Migrate local SDL2 copy to Git submodule

.gitmodules

@@ -4,3 +4,6 @@
 [submodule "data"]
 	path = data
 	url = https://gitlab.com/cjhoward/antkeeper-data.git
+[submodule "lib\\SDL2"]
+	path = lib\\SDL2
+	url = https://github.com/cjhoward/SDL2.git

CMakeLists.txt

@@ -143,7 +143,7 @@ set(SDL2_LIBRARIES
 	uuid
 )
 ExternalProject_Add(SDL2-project
-	SOURCE_DIR ${PROJECT_SOURCE_DIR}/lib/SDL2-2.0.5
+	SOURCE_DIR ${PROJECT_SOURCE_DIR}/lib/SDL2
 	CMAKE_ARGS
 	"-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"
 	"-DCMAKE_INSTALL_PREFIX=${SDL2_INSTALL_DIR}"

lib/SDL2

@@ -0,0 +1 @@
+Subproject commit 6f6d0abad18a8a1bb8131d2676a4ed6e606b1ccf