SDLWindow links to and export invalid target names

[khardix]

When building the GPUEngine addons (geAd), the configuration fails with following error(s):

$ cd GPUEngine
$ cmake . -DGPUENGINE_BUILD_geAd=ON
...
CMake Error at geAd/SDLWindow/CMakeLists.txt:34 (add_library):
  Target "SDLWindow" links to target "SDL2::SDL2" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?


CMake Error at geAd/SDLWindow/CMakeLists.txt:34 (add_library):
  Target "SDLWindow" links to target "SDL2::SDL2main" but the target was not
  found.  Perhaps a find_package() call is missing for an IMPORTED target, or
  an ALIAS target is missing?


CMake Error at geAd/SDLOrbitManipulator/CMakeLists.txt:32 (add_library):
  Target "SDLOrbitManipulator" links to target "SDL2::SDL2" but the target
  was not found.  Perhaps a find_package() call is missing for an IMPORTED
  target, or an ALIAS target is missing?


CMake Error at geAd/SDLOrbitManipulator/CMakeLists.txt:32 (add_library):
  Target "SDLOrbitManipulator" links to target "SDL2::SDL2main" but the
  target was not found.  Perhaps a find_package() call is missing for an
  IMPORTED target, or an ALIAS target is missing?
...

The SDL2 library is installed and found (the SDL2_FOUND variable is set).

---

After poking in the module search definitions, it seems that FindSDL2.cmake produces only target SDL2, not SDL2::SDL2 and SDL2::SDL2main – at least on my machine :)

After skype discussion with @forry, the suggested fix is to set External_deps_Export and External_libs in geAd/SDLWindow/CMakeLists.txt to only SDL2. I can confirm this fixes the issue on my machine.