[CMake] Why isn't target_link_libraries not enough in some cases?

[Francis Giraldeau]

> Hi,
>
> I am fetching and building SDL2 using FetchContent and then using
> target_link_libraries(MyExe SDL2) in the hopes that the required include
> directories and libraries will be added populated properly. The example
> project can be found here:
>
> https://github.com/samaursa/cmake_fetch_content_SDL2
>
> Unfortunately, it seems that I have to manually specify the include
> directories. Although target_link_directories(...) does take care of the
> linking against the correct libraries. When it comes to my own CMake
> enabled libraries, I do not have to specify the include directories.

The SDL add_library() only specify the include directory for install
target:

  target_link_libraries(SDL2 ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
  target_include_directories(SDL2 PUBLIC $<INSTALL_INTERFACE:include/SDL2>)

They themselves include directories using the old global includes instead
of target specific includes:

  include_directories(${SDL2_BINARY_DIR}/include ${SDL2_SOURCE_DIR}/include)

There should be a target_include_directories(SDL2 PUBLIC
<$BUILD_INTERFACE:${SDL2_SOURCE_DIR}/include), but that must be in the same
CMakeLists that defines the library, which require changing the SDL's
CMakeLists.

So, one solution could be to create an imported target to attach this path
and l ink to it instead:

  add_library(SDL2x INTERFACE IMPORTED)
  set_target_properties(SDL2x PROPERTIES INTERFACE_INCLUDE_DIRECTORIES
"${sdl2_SOURCE_DIR}/include")
  set_target_properties(SDL2x PROPERTIES INTERFACE_LINK_LIBRARIES SDL2)

Tested with CMake 3.11. However, to make it work, I had to add OpenGL:

  find_package(OpenGL REQUIRED)
  target_link_libraries(testExe SDL2x OpenGL::GL)

Install fails, but that's another issue (the symlink is created in the
wrong directory) and this should be reported to SDL I guess.

CMake Error at _deps/sdl2-build/cmake_install.cmake:188 (file):
  file INSTALL cannot find
  "[...]/build/_deps/sdl2-build/libSDL2.so".
Call Stack (most recent call first):
  cmake_install.cmake:42 (include)

Francis
-- 
Francis Giraldeau
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