use in-tree SDL

.gitmodules

@@ -0,0 +1,4 @@
+[submodule "SDL"]
+	path = SDL
+	url = https://github.com/libsdl-org/SDL.git
+	branch = 0280d8249ad1c8ae8f699b047984880a084812e5

CMakeLists.txt

@@ -95,19 +95,19 @@ if(UNIX)
   add_compile_options("$<$<COMPILE_LANGUAGE:CXX>:-Wno-multichar;${BITS};${EXTRA_CXX_FLAGS}>")
   add_compile_options("$<$<COMPILE_LANGUAGE:C>:${BITS}>")
 
-  find_package(SDL2 REQUIRED)
+  add_subdirectory(SDL EXCLUDE_FROM_ALL)
   # Some versions of the SDL2 find_package set SDL2_INCLUDE_DIR and some set a plural SDL2_INCLUDE_DIRS. Check both.
-  message("SDL2 Include Dir is ${SDL2_INCLUDE_DIR} ${SDL2_INCLUDE_DIRS}")
+  message("SDL2 Include Dir is SDL/include")
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
   message("Building for Linux")
   add_compile_definitions(__LINUX__ _USE_OGL_ACTIVE_TEXTURES PRIMARY_HOG=\"d3-linux.hog\")
-  set(PLATFORM_INCLUDES "lib/linux" ${SDL2_INCLUDE_DIR} ${SDL2_INCLUDE_DIRS})
+  set(PLATFORM_INCLUDES "lib/linux" "SDL/include")
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
   message("Building for MAC OSX")
   add_compile_definitions(__LINUX__ MACOSX=1 _USE_OGL_ACTIVE_TEXTURES PRIMARY_HOG=\"d3-osx.hog\")
-  set(PLATFORM_INCLUDES "lib/linux" ${SDL2_INCLUDE_DIR} ${SDL2_INCLUDE_DIRS})
+  set(PLATFORM_INCLUDES "lib/linux" "SDL/include")
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Windows")
   # Windows.h defines to avoid as many issues as possible.
   add_compile_definitions(WIN32_LEAN_AND_MEAN NOMINMAX NODRAWTEXT NOBITMAP NOMCX NOSERVICE PRIMARY_HOG=\"d3-win.hog\"