CMake: Fix SDL2 include directory

Since SDL includes its own CMake config since version 2.0.4, which takes
precedence over our FindSDL2.cmake script if present. This is mostly a
good thing except for the SDL2 config defining SDL2_INCLUDE_DIRS only
whereas we used SDL2_INCLUDE_DIR. Fix that mismatch.

CMakeLists.txt

@@ -599,7 +599,7 @@ find_package(PythonInterp)
 if(ARX_HAVE_SDL AND ARX_HAVE_OPENGL AND NOT WIN32 AND NOT APPLE)
 	set(old_CMAKE_REQUIRED_INCLUDES)
 	if(ARX_HAVE_SDL2)
-		set(CMAKE_REQUIRED_INCLUDES ${SDL2_INCLUDE_DIR})
+		set(CMAKE_REQUIRED_INCLUDES ${SDL2_INCLUDE_DIRS})
 	else()
 		set(CMAKE_REQUIRED_INCLUDES ${SDL_INCLUDE_DIR})
 	endif()
@@ -1289,7 +1289,7 @@ if(ARX_HAVE_SDL)
 		list(APPEND WINDOW_SOURCES ${WINDOW_SDL2_SOURCES})
 		list(APPEND INPUT_SOURCES ${INPUT_SDL2_SOURCES})
 		list(APPEND ARX_LIBRARIES ${SDL2_LIBRARIES})
-		include_directories(SYSTEM ${SDL2_INCLUDE_DIR})
+		include_directories(SYSTEM ${SDL2_INCLUDE_DIRS})
 	elseif(ARX_HAVE_SDL1)
 		list(APPEND WINDOW_SOURCES ${WINDOW_SDL1_SOURCES})
 		list(APPEND INPUT_SOURCES ${INPUT_SDL1_SOURCES})

cmake/FindSDL2.cmake

@@ -162,6 +162,7 @@ find_package_handle_standard_args(SDL2 REQUIRED_VARS SDL2_LIBRARY SDL2_INCLUDE_D
 
 if(SDL2_LIBRARY)
 
+	set(SDL2_INCLUDE_DIRS ${SDL2_INCLUDE_DIR})
 	set(SDL2_LIBRARIES ${SDL2_LIBRARY})
 
 	# If we're linking against a static libSDL2 we also need to link against some other