quiet -Wmissing-prototypes warnings, and enable this warning by default for C with gcc.

helps for finding unused functions and making functions static, also did some minor code cleanup.

source/blender/python/intern/bpy_app_handlers.c

@@ -37,6 +37,8 @@
 #include "bpy_rna.h"
 #include "bpy_app_handlers.h"
 
+#include "BPY_extern.h"
+
 void bpy_app_generic_callback(struct Main *main, struct ID *id, void *arg);
 
 static PyTypeObject BlenderAppCbType;