Add SDL 2.0.1 headers for macOS PPC

code/libs/macosx/libSDL2-2.0.0.dylib has 2.0.8 for x86 and x86_64 and
2.0.1 for PPC. Add 2.0.1 headers for PPC with modifed SDL_platform.h to
allow compiling using macOS 10.5 SDK. Using separate headers allows the
engine to check the SDL version for enabling newer SDL features.

code/SDL2/include-macppc/SDL_revision.h

@@ -0,0 +1,2 @@
+#define SDL_REVISION "hg-7890:c031abe0b287"
+#define SDL_REVISION_NUMBER 7890