Cygwin/X: Turn off the bad-function-cast warning for XWin code

Turn off the bad-function-cast warning for the XWin code, it's near impossible
to write code which uses Win32 API calls that doesn't trigger this warning

For example, SendMessage(WM_SETICON) returns an LRESULT, but we are supposed to
know this is safe to cast this result to a HICON, which gcc considers a non-matching
type.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

hw/xwin/Makefile.am

@@ -162,7 +162,8 @@ AM_YFLAGS = -d
 AM_LFLAGS = -i
 AM_CFLAGS = -DHAVE_XWIN_CONFIG_H $(DIX_CFLAGS) \
             $(XWINMODULES_CFLAGS) \
-            -I$(top_srcdir)
+            -I$(top_srcdir) \
+            -Wno-bad-function-cast
 
 MAN_SRCS = XWin.man.pre XWinrc.man.pre