Impl specific functionality

[John Emmas <johne53 tiscali co uk>]

Suppose I'm writing an app that can either be built with gtk-x11 or gtk-win32.  And suppose there's a 
function that I'd only ever need to call in the X11 version (XSetErrorHandler() is probably a good example).  
Is there a #define or something that I can use at compile time (or even something I could call at run time) - 
for example:-

#if GDK_IMPL_X11
   XSetErrorHandler ( some_func );
#elif GDK_IMPL_WIN32
   // Do some Windows specific stuff
#endif

Of course I can define my own but I just wondered if there's something already available?

John