Use consistent identifiers for the various platforms we support.

Make sure every source file includes SDL_config.h, so the proper system
headers are chosen.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401406

include/SDL_config.h

@@ -25,6 +25,8 @@
 
 /* This is the minimal configuration that can be used to build SDL */
 
+#include "SDL_platform.h"
+
 #include <stdarg.h>
 
 typedef signed char int8_t;