image_background() is declared in dispextern.h with a return type of 
`unsigned long'.  But it is defined in image.c with a return type of 
RGB_PIXEL_COLOR, where the latter is a platform-dependent macro defined 
in image.c.

By luck, this doesn't cause any problems in the currently-supported 
platforms.  But it will cause problems in the cygw32 build on 64-bit 
Cygwin (which is currently under development).  The attached patch fixes 
this by moving the definition of RGB_PIXEL_COLOR to dispextern.h and 
changing the return type in the declaration.

OK to apply?

Ken