Add SDL2 to main CMakeLists.txt.

Also expose SDL_LINK_DIR. This is required because the main target
inherits some of SDL2 extra dependcies (DirectX) for some reason.

libs/SDL2-2.0.0/CMakeLists.txt

@@ -756,7 +756,9 @@ elseif(WINDOWS)
     if(HAVE_D3D_H OR HAVE_DDRAW_H OR HAVE_DSOUND_H OR HAVE_DINPUT_H OR HAVE_XAUDIO2_H)
       set(HAVE_DIRECTX TRUE)
       # TODO: change $ENV{DXSDL_DIR} to get the path from the include checks
-      link_directories($ENV{DXSDK_DIR}\\lib\\${PROCESSOR_ARCH})
+      set(SDL_LINK_DIR $ENV{DXSDK_DIR}\\lib\\${PROCESSOR_ARCH})
+      set(SDL_LINK_DIR ${SDL_LINK_DIR} PARENT_SCOPE)
+      link_directories(${SDL_LINK_DIR})
       include_directories($ENV{DXSDK_DIR}\\Include)
     endif()
     set(CMAKE_REQUIRED_FLAGS)
@@ -1159,6 +1161,7 @@ if(SDL_SHARED)
   endif(UNIX)
  set(_INSTALL_LIBS "SDL2" ${_INSTALL_LIBS})
  target_link_libraries(SDL2 ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
+ target_include_directories(SDL2 PUBLIC include)
 endif(SDL_SHARED)
 
 if(SDL_STATIC)
@@ -1168,6 +1171,7 @@ if(SDL_STATIC)
   # libraries - do we need to consider this?
   set(_INSTALL_LIBS "SDL2-static" ${_INSTALL_LIBS})
   target_link_libraries(SDL2-static ${EXTRA_LIBS} ${EXTRA_LDFLAGS})
+  target_include_directories(SDL2-static PUBLIC include)
 endif(SDL_STATIC)
 
 ##### Installation targets #####