modules: make FindSDL2 delegate to SDL2's own config file, if possible.

Now that the Find module doesn't need to get copied to all downstream
repositories, I can finally work on it in peace.

doc/changelog.dox

@@ -1100,6 +1100,9 @@ See also:
     dependencies in a way that doesn't clash with modules of the same name
     potentially installed by other software, meaning the user doesn't need to
     copy and carry them along anymore (see [mosra/magnum#436](https://github.com/mosra/magnum/issues/436))
+-   The `FindSDL2.cmake` module now delegates to SDL's own CMake config file,
+    if available, especially to ensure all dependencies are correctly linked on
+    static builds (see [mosra/magnum#553](https://github.com/mosra/magnum/issues/553))
 
 @subsection changelog-latest-bugfixes Bug fixes
 

modules/FindSDL2.cmake

@@ -43,11 +43,41 @@
 #   DEALINGS IN THE SOFTWARE.
 #
 
-# If we have a CMake subproject, use the targets directly. I'd prefer the
-# static variant, however SDL2 defines its own SDL2::SDL2 alias for only the
-# dynamic variant since https://github.com/libsdl-org/SDL/pull/4074 and so I'm
-# forced to use that, if available.
-if(TARGET SDL2)
+# SDL2 installs CMake package config files which handle dependencies in case
+# it's built statically. Try to find first, quietly, so it doesn't print
+# loud messages when it's not found, since that's okay. If the unprefixed SDL2
+# targets already exist, it means we're using it through a CMake subproject --
+# don't attempt to find the package in that case.
+#
+# Don't do this on Emscripten -- there the config file is broken if
+# EMSCRIPTEN_SYSROOT isn't defined, adding /include/SDL2 as an include path.
+# Plus, it wants to pass -sUSE_SDL=2, which we definitely *do not* want here.
+if(NOT CORRADE_TARGET_EMSCRIPTEN AND NOT TARGET SDL2 AND NOT TARGET SDL2-static)
+    find_package(SDL2 CONFIG QUIET)
+endif()
+
+# If either a SDL2 config file was found or we have a CMake subproject, use the
+# targets directly. I'd prefer the static variant if there's a choice, however
+# SDL2 defines its own SDL2::SDL2 alias for only the dynamic variant since
+# https://github.com/libsdl-org/SDL/pull/4074 and so I'm forced to use that, if
+# available.
+if(TARGET SDL2::SDL2 OR TARGET SDL2::SDL2-static OR TARGET SDL2 OR TARGET SDL2-static)
+    # The static build is a separate target for some reason. I wonder HOW that
+    # makes more sense than just having a build-time option for static/shared
+    # and use the same name for both. Are all depending projects supposed to
+    # branch on it like this?!
+    if(TARGET SDL2::SDL2)
+        set(_SDL2_TARGET SDL2::SDL2)
+        set(_SDL2_DYNAMIC ON)
+    elseif(TARGET SDL2::SDL2-static)
+        set(_SDL2_TARGET SDL2::SDL2-static)
+    elseif(TARGET SDL2)
+        set(_SDL2_TARGET SDL2)
+        set(_SDL2_DYNAMIC ON)
+    elseif(TARGET SDL2-static)
+        set(_SDL2_TARGET SDL2-static)
+    endif()
+
     # In case we don't have https://github.com/libsdl-org/SDL/pull/4074 yet,
     # do the alias ourselves.
     if(NOT TARGET SDL2::SDL2)
@@ -56,47 +86,30 @@ if(TARGET SDL2)
         # properties (which are impossible to track of) and then attempting to
         # rebuild them into a new target.
         add_library(SDL2::SDL2 INTERFACE IMPORTED)
-        set_target_properties(SDL2::SDL2 PROPERTIES INTERFACE_LINK_LIBRARIES SDL2)
+        set_target_properties(SDL2::SDL2 PROPERTIES INTERFACE_LINK_LIBRARIES ${_SDL2_TARGET})
     endif()
 
     # Just to make FPHSA print some meaningful location, nothing else. Not
-    # using the INTERFACE_INCLUDE_DIRECTORIES as that contains
-    # $<BUILD_INTERFACE and looks ugly in the output. Funnily enough, the
-    # BUILD_INTERFACE thing works here without having to override it with
-    # custom-found paths like I do in FindAssimp and elsewhere. Needs further
-    # investigation.
-    get_target_property(_SDL2_SOURCE_DIR SDL2 SOURCE_DIR)
+    # using the INTERFACE_INCLUDE_DIRECTORIES for the CMake subproject case as
+    # that contains $<BUILD_INTERFACE and looks ugly in the output. Funnily
+    # enough, the BUILD_INTERFACE thing works here without having to override
+    # it with custom-found paths like I do in FindAssimp and elsewhere. Needs
+    # further investigation.
     include(FindPackageHandleStandardArgs)
-    find_package_handle_standard_args("SDL2" DEFAULT_MSG _SDL2_SOURCE_DIR)
+    if(TARGET SDL2 OR TARGET SDL2-static)
+        get_target_property(_SDL2_SOURCE_DIR ${_SDL2_TARGET} SOURCE_DIR)
+        find_package_handle_standard_args("SDL2" DEFAULT_MSG _SDL2_SOURCE_DIR)
+    else()
+        get_target_property(_SDL2_INTERFACE_INCLUDE_DIRECTORIES ${_SDL2_TARGET} INTERFACE_INCLUDE_DIRECTORIES)
+        find_package_handle_standard_args("SDL2" DEFAULT_MSG _SDL2_INTERFACE_INCLUDE_DIRECTORIES)
+    endif()
 
-    if(CORRADE_TARGET_WINDOWS)
+    if(CORRADE_TARGET_WINDOWS AND _SDL2_DYNAMIC)
         # .dll is in LOCATION, .lib is in IMPLIB. Yay, useful!
         get_target_property(SDL2_DLL_DEBUG SDL2 IMPORTED_LOCATION_DEBUG)
         get_target_property(SDL2_DLL_RELEASE SDL2 IMPORTED_LOCATION_RELEASE)
     endif()
 
-    return()
-
-# The static build is a separate target for some reason. I wonder HOW that
-# makes more sense than just having a build-time option for static/shared and
-# use the same name for both. Are all depending projects supposed to branch on
-# it like this?!
-elseif(TARGET SDL2-static)
-    # The target should not be defined by SDL itself. If it is, that's from us.
-    if(NOT TARGET SDL2::SDL2)
-        # Aliases of (global) targets are only supported in CMake 3.11, so we
-        # work around it by this. This is easier than fetching all possible
-        # properties (which are impossible to track of) and then attempting to
-        # rebuild them into a new target.
-        add_library(SDL2::SDL2 INTERFACE IMPORTED)
-        set_target_properties(SDL2::SDL2 PROPERTIES INTERFACE_LINK_LIBRARIES SDL2-static)
-    endif()
-
-    # Just to make FPHSA print some meaningful location, nothing else
-    get_target_property(_SDL2_SOURCE_DIR SDL2-static SOURCE_DIR)
-    include(FindPackageHandleStandardArgs)
-    find_package_handle_standard_args("SDL2" DEFAULT_MSG _SDL2_SOURCE_DIR)
-
     return()
 endif()