gmain: Add G_PID_FORMAT

This will be useful for printing out GPids in printf()-style functions.

https://bugzilla.gnome.org/show_bug.cgi?id=767765

glib/glibconfig.h.win32.in

@@ -242,6 +242,7 @@ typedef unsigned __int64 guintptr;
  * not a process identifier.
  */
 typedef void * GPid;
+#define G_PID_FORMAT "p"
 
 #define GLIB_SYSDEF_AF_UNIX 1
 #define GLIB_SYSDEF_AF_INET 2