CMake: make OpenGL_GL_PREFERENCE settable from outside.

CMakeLists.txt

@@ -212,7 +212,13 @@ endif()
 # Check dependencies
 if(WITH_GL)
     if(NOT TARGET_GLES OR TARGET_DESKTOP_GLES)
-        set(OpenGL_GL_PREFERENCE GLVND) # since CMake 3.11
+        # OpenGL library preference. Prefer to use GLVND, since that's the
+        # better approach nowadays, but allow the users to override it from
+        # outside in case it is broken for some reason (Nvidia drivers in
+        # Debian's testing (Buster) -- reported on 2019-04-09).
+        if(NOT CMAKE_VERSION VERSION_LESS 3.10 AND NOT OpenGL_GL_PREFERENCE)
+            set(OpenGL_GL_PREFERENCE GLVND)
+        endif()
         find_package(OpenGL REQUIRED)
     elseif(TARGET_GLES2)
         find_package(OpenGLES2 REQUIRED)

doc/changelog.dox

@@ -267,6 +267,10 @@ See also:
 -   Updated bundled GL headers to properly define the `GL_VERSION_*` macros to
     avoid conflicting `GLintptr` and `GLsizeiptr` type definitions on Windows
     when using Magnum together with Qt
+-   It's now possible to override CMake's
+    [OpenGL_GL_PREFERENCE](https://cmake.org/cmake/help/v3.11/module/FindOpenGL.html#linux-specific)
+    from outside on systems where GLVND doesn't work properly (before the build
+    scripts and `FindMagnum.cmake` always forced GLVND)
 
 @subsection changelog-latest-bugfixes Bug fixes
 

modules/FindMagnum.cmake

@@ -260,6 +260,14 @@ foreach(_magnumFlag ${_magnumFlags})
     endif()
 endforeach()
 
+# OpenGL library preference. Prefer to use GLVND, since that's the better
+# approach nowadays, but allow the users to override it from outside in case
+# it is broken for some reason (Nvidia drivers in Debian's testing (Buster) --
+# reported on 2019-04-09).
+if(NOT CMAKE_VERSION VERSION_LESS 3.10 AND NOT OpenGL_GL_PREFERENCE)
+    set(OpenGL_GL_PREFERENCE GLVND)
+endif()
+
 # Base Magnum library
 if(NOT TARGET Magnum::Magnum)
     add_library(Magnum::Magnum UNKNOWN IMPORTED)
@@ -618,12 +626,13 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 # our own EGL find module, which makes things simpler. The
                 # upstream FindOpenGL is anything but simple. Also can't use
                 # OpenGL_OpenGL_FOUND, because that one is set also if GLVND is
-                # *not* found. WTF.
+                # *not* found. WTF. Also can't just check for
+                # OPENGL_opengl_LIBRARY because that's set even if
+                # OpenGL_GL_PREFERENCE is explicitly set to LEGACY.
                 if(MAGNUM_TARGET_GL)
                     if(CORRADE_TARGET_UNIX AND NOT CORRADE_TARGET_APPLE AND (NOT MAGNUM_TARGET_GLES OR MAGNUM_TARGET_DESKTOP_GLES))
-                        set(OpenGL_GL_PREFERENCE GLVND)
                         find_package(OpenGL)
-                        if(OPENGL_opengl_LIBRARY)
+                        if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
                             set_property(TARGET Magnum::${_component} APPEND
                             PROPERTY INTERFACE_LINK_LIBRARIES OpenGL::GLX)
                         endif()
@@ -656,12 +665,13 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 # our own EGL find module, which makes things simpler. The
                 # upstream FindOpenGL is anything but simple. Also can't use
                 # OpenGL_OpenGL_FOUND, because that one is set also if GLVND is
-                # *not* found. WTF.
+                # *not* found. WTF. Also can't just check for
+                # OPENGL_opengl_LIBRARY because that's set even if
+                # OpenGL_GL_PREFERENCE is explicitly set to LEGACY.
                 if(MAGNUM_TARGET_GL)
                     if(CORRADE_TARGET_UNIX AND NOT CORRADE_TARGET_APPLE AND (NOT MAGNUM_TARGET_GLES OR MAGNUM_TARGET_DESKTOP_GLES))
-                        set(OpenGL_GL_PREFERENCE GLVND)
                         find_package(OpenGL)
-                        if(OPENGL_opengl_LIBRARY)
+                        if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
                             set_property(TARGET Magnum::${_component} APPEND
                             PROPERTY INTERFACE_LINK_LIBRARIES OpenGL::GLX)
                         endif()
@@ -683,10 +693,11 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 # With GLVND (since CMake 3.11) we need to explicitly link to
                 # GLX because libOpenGL doesn't provide it. Also can't use
                 # OpenGL_OpenGL_FOUND, because that one is set also if GLVND is
-                # *not* found. WTF.
-                set(OpenGL_GL_PREFERENCE GLVND)
+                # *not* found. WTF. Also can't just check for
+                # OPENGL_opengl_LIBRARY because that's set even if
+                # OpenGL_GL_PREFERENCE is explicitly set to LEGACY.
                 find_package(OpenGL)
-                if(OPENGL_opengl_LIBRARY)
+                if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
                     set_property(TARGET Magnum::${_component} APPEND PROPERTY
                         INTERFACE_LINK_LIBRARIES OpenGL::GLX)
                 endif()
@@ -736,10 +747,11 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 # With GLVND (since CMake 3.11) we need to explicitly link to
                 # GLX because libOpenGL doesn't provide it. Also can't use
                 # OpenGL_OpenGL_FOUND, because that one is set also if GLVND is
-                # *not* found. If GLVND is not used, link to X11 instead.
-                set(OpenGL_GL_PREFERENCE GLVND)
+                # *not* found. If GLVND is not used, link to X11 instead. Also
+                # can't just check for OPENGL_opengl_LIBRARY because that's set
+                # even if OpenGL_GL_PREFERENCE is explicitly set to LEGACY.
                 find_package(OpenGL)
-                if(OPENGL_opengl_LIBRARY)
+                if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
                     set_property(TARGET Magnum::${_component} APPEND PROPERTY
                         INTERFACE_LINK_LIBRARIES OpenGL::GLX)
                 else()
@@ -780,10 +792,11 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 # imported target. Otherwise (and also on all systems except
                 # Linux) link to the classic libGL. Can't use
                 # OpenGL_OpenGL_FOUND, because that one is set also if GLVND is
-                # *not* found. WTF.
-                set(OpenGL_GL_PREFERENCE GLVND)
+                # *not* found. WTF. Also can't just check for
+                # OPENGL_opengl_LIBRARY because that's set even if
+                # OpenGL_GL_PREFERENCE is explicitly set to LEGACY.
                 find_package(OpenGL REQUIRED)
-                if(OPENGL_opengl_LIBRARY)
+                if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
                     set_property(TARGET Magnum::${_component} APPEND PROPERTY
                         INTERFACE_LINK_LIBRARIES OpenGL::OpenGL)
                 else()

src/Magnum/GL/CMakeLists.txt

@@ -220,8 +220,10 @@ if(NOT TARGET_GLES OR TARGET_DESKTOP_GLES)
     # If the GLVND library (CMake 3.11+) was found, link to the imported
     # target. Otherwise (and also on all systems except Linux) link to the
     # classic libGL. Can't use OpenGL_OpenGL_FOUND, because that one is set
-    # also if GLVND is *not* found. WTF.
-    if(OPENGL_opengl_LIBRARY)
+    # also if GLVND is *not* found. WTF. Also can't just check for
+    # OPENGL_opengl_LIBRARY because that's set even if OpenGL_GL_PREFERENCE is
+    # explicitly set to LEGACY.
+    if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
         target_link_libraries(MagnumGL PUBLIC OpenGL::OpenGL)
     else()
         target_link_libraries(MagnumGL PUBLIC ${OPENGL_gl_LIBRARY})
@@ -308,8 +310,10 @@ if(BUILD_TESTS)
         # If the GLVND library (CMake 3.11+) was found, link to the imported
         # target. Otherwise (and also on all systems except Linux) link to the
         # classic libGL. Can't use OpenGL_OpenGL_FOUND, because that one is set
-        # also if GLVND is *not* found. WTF.
-        if(OPENGL_opengl_LIBRARY)
+        # also if GLVND is *not* found. WTF. Also can't just check for
+        # OPENGL_opengl_LIBRARY because that's set even if OpenGL_GL_PREFERENCE
+        # is explicitly set to LEGACY.
+        if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
             target_link_libraries(MagnumGLTestLib PUBLIC OpenGL::OpenGL)
         else()
             target_link_libraries(MagnumGLTestLib PUBLIC ${OPENGL_gl_LIBRARY})

src/Magnum/Platform/CMakeLists.txt

@@ -106,8 +106,10 @@ if((WITH_GLFWAPPLICATION OR WITH_SDL2APPLICATION) AND TARGET_GL)
         # to link to GLX explicitly. Otherwise (and also on all systems except
         # Linux) the transitive dependency to classic GL lib from MagnumGL is
         # enough. Can't use OpenGL_OpenGL_FOUND, because that one is set also
-        # if GLVND is *not* found. WTF.
-        if(OPENGL_opengl_LIBRARY)
+        # if GLVND is *not* found. WTF. Also can't just check for
+        # OPENGL_opengl_LIBRARY because that's set even if OpenGL_GL_PREFERENCE
+        # is explicitly set to LEGACY.
+        if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
             set(MagnumSomeContext_LIBRARY OpenGL::GLX)
         endif()
     elseif(MAGNUM_TARGET_GLES AND NOT CORRADE_TARGET_EMSCRIPTEN)