Platform: properly link GLX apps to libGLX on an GLES + EGL build.

It fails on a linker error otherwise. There's a bit of annoying logic
needed for pre-GLVND systems as there it's not possible to link to GLX
without dragging the whole libGL in as well, causing bad conflicts with
libGLES. Which means I can't test this on the CI yet as there CMake is
forced to version 3.5 and finding GLVND is only in 3.10.

modules/FindMagnum.cmake

@@ -810,6 +810,13 @@ foreach(_component ${Magnum_FIND_COMPONENTS})
                 # *not* found. WTF. Also can't just check for
                 # OPENGL_opengl_LIBRARY because that's set even if
                 # OpenGL_GL_PREFERENCE is explicitly set to LEGACY.
+                #
+                # If MAGNUM_TARGET_GLES and MAGNUM_TARGET_EGL is set, these
+                # applications can be built only if GLVND is available as
+                # otherwise there would be a conflict between libGL and
+                # libGLES. Thus, if GLVND is not available, it won't link
+                # libGLX here, but that shouldn't be a problem since the
+                # application library won't exist either.
                 find_package(OpenGL)
                 if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
                     set_property(TARGET Magnum::${_component} APPEND PROPERTY

package/ci/circleci.yml

@@ -374,6 +374,9 @@ jobs:
       CMAKE_CXX_FLAGS: --coverage
       LCOV_EXTRA_OPTS: --gcov-tool /usr/bin/gcov-4.8
       CONFIGURATION: Debug
+      # TODO enable also GlxApplication, WindowlessGlxApplication and
+      # GlxContext once CMake 3.10+ is used to test correct libGLES + libGLX
+      # linking
       EXTRA_OPTS: >
         -DMAGNUM_WITH_XEGLAPPLICATION=ON
         -DMAGNUM_WITH_WINDOWLESSEGLAPPLICATION=ON
@@ -403,6 +406,9 @@ jobs:
       CMAKE_CXX_FLAGS: --coverage
       LCOV_EXTRA_OPTS: --gcov-tool /usr/bin/gcov-4.8
       CONFIGURATION: Debug
+      # TODO enable also GlxApplication, WindowlessGlxApplication and
+      # GlxContext once CMake 3.10+ is used to test correct libGLES + libGLX
+      # linking
       EXTRA_OPTS: >
         -DMAGNUM_WITH_XEGLAPPLICATION=ON
         -DMAGNUM_WITH_WINDOWLESSEGLAPPLICATION=ON

src/Magnum/Platform/CMakeLists.txt

@@ -422,6 +422,17 @@ if(MAGNUM_WITH_GLXAPPLICATION)
         message(FATAL_ERROR "X11 library, required by some applications, was not found. Set MAGNUM_WITH_*X*APPLICATION to OFF to skip building them.")
     endif()
 
+    # If we're targeting GLES and EGL, only the libGLES library is looked for
+    # by default. Here we need libGLX from GLVND in addition. If GLVND is not
+    # available however (or CMake < 3.10 is used), fail, as GLX symbols are
+    # only in libGL in that case, which would conflict with libGLES.
+    if(MAGNUM_TARGET_GLES AND MAGNUM_TARGET_EGL)
+        find_package(OpenGL)
+        if(NOT OpenGL_FOUND OR NOT OPENGL_opengl_LIBRARY OR NOT OpenGL_GL_PREFERENCE STREQUAL GLVND)
+            message(SEND_ERROR "Without GLVND, GlxApplication is available only if MAGNUM_TARGET_EGL is disabled. Use XEglApplication on EGL instead.")
+        endif()
+    endif()
+
     set(MagnumGlxApplication_SRCS
         GlxApplication.cpp
         $<TARGET_OBJECTS:MagnumAbstractXApplication>
@@ -446,6 +457,9 @@ if(MAGNUM_WITH_GLXAPPLICATION)
     # Can't use OpenGL_OpenGL_FOUND, because that one is set 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.
+    #
+    # See also the logic above for preventing libGL and libGLES conflicts. If
+    # there's no conflict, this branch is taken and libGLX is linked.
     if(OPENGL_opengl_LIBRARY AND OpenGL_GL_PREFERENCE STREQUAL GLVND)
         target_link_libraries(MagnumGlxApplication PUBLIC OpenGL::GLX)
     endif()
@@ -479,6 +493,10 @@ if(MAGNUM_WITH_XEGLAPPLICATION)
         message(FATAL_ERROR "X11 library, required by some applications, was not found. Set MAGNUM_WITH_*X*APPLICATION to OFF to skip building them.")
     endif()
 
+    # Unlike with GlxApplication above, here no conflict between libGLES and
+    # libGL can happen -- we only either link libGLES and libEGL or libGL and
+    # libEGL.
+
     set(MagnumXEglApplication_SRCS
         XEglApplication.cpp
         $<TARGET_OBJECTS:MagnumAbstractXApplication>
@@ -557,6 +575,17 @@ if(MAGNUM_WITH_WINDOWLESSGLXAPPLICATION)
         message(FATAL_ERROR "X11 library, required by some applications, was not found. Set MAGNUM_WITH_*X*APPLICATION to OFF to skip building them.")
     endif()
 
+    # If we're targeting GLES and EGL, only the libGLES library is looked for
+    # by default. Here we need libGLX from GLVND in addition. If GLVND is not
+    # available however (or CMake < 3.10 is used), fail, as GLX symbols are
+    # only in libGL in that case, which would conflict with libGLES.
+    if(MAGNUM_TARGET_GLES AND MAGNUM_TARGET_EGL)
+        find_package(OpenGL)
+        if(NOT OpenGL_FOUND OR NOT OPENGL_opengl_LIBRARY OR NOT OpenGL_GL_PREFERENCE STREQUAL GLVND)
+            message(SEND_ERROR "Without GLVND, WindowlessGlxApplication is available only if MAGNUM_TARGET_EGL is disabled. Use WindowlessEglApplication on EGL instead.")
+        endif()
+    endif()
+
     set(MagnumWindowlessGlxApplication_SRCS
         WindowlessGlxApplication.cpp
         $<TARGET_OBJECTS:MagnumGlxContextObjects>)