Refactor all #define SUPPORT_* into a config.h

That way, a user needs only to touch a single file to configure what
features raylib is built with.
Include guards are left out intentionally, because config.h should only
be included in source files, not headers.

Later on, config.h can also define the raylib version (#461).

src/utils.h

@@ -32,7 +32,9 @@
     #include <android/asset_manager.h>      // Required for: AAssetManager
 #endif
 
-#define SUPPORT_SAVE_PNG
+#ifndef SUPPORT_SAVE_PNG
+#define SUPPORT_SAVE_PNG 1
+#endif
 
 //----------------------------------------------------------------------------------
 // Some basic Defines
@@ -74,4 +76,4 @@ FILE *android_fopen(const char *fileName, const char *mode);    // Replacement f
 }
 #endif
 
-#endif // UTILS_H
+#endif // UTILS_H