Reapply SDL2 modifications

The prevents Windows builds from leaking SDL2's exports

external/SDL2/CMakeLists.txt

@@ -1843,6 +1843,7 @@ endif()
 if(SDL_STATIC)
   set (BUILD_SHARED_LIBS FALSE)
   add_library(SDL2-static STATIC ${SOURCE_FILES})
+  target_compile_definitions(SDL2-static PRIVATE SDL_STATIC)
   if (NOT SDL_SHARED OR NOT WIN32)
     set_target_properties(SDL2-static PROPERTIES OUTPUT_NAME "SDL2")
     # Note: Apparently, OUTPUT_NAME must really be unique; even when

external/SDL2/include/begin_code.h

@@ -58,6 +58,8 @@
 #   else
 #    define DECLSPEC    __declspec(dllimport)
 #   endif
+#  elif defined(SDL_STATIC)
+#   define DECLSPEC
 #  else
 #   define DECLSPEC __declspec(dllexport)
 #  endif