Remove system-specific -I switch from Cygwin-w32 build

Suggested by Eli Zaretskii in discussion of bug#18302.

* src/image.c [HAVE_NTGUI]: Remove the unused macro
PIXEL_ALREADY_TYPEDEFED.  On Cygwin, include "noX/xpm.h" instead of
"X11/xpm.h".

* configure.ac [CYGWIN && HAVE_W32]: Change the xpm test to use
"noX/xpm.h".  Don't add a -I switch to CPPFLAGS.

1 files changed, 6 insertions, 4 deletions

diff --git a/src/image.c b/src/image.c
index 38866e08345..657852b1a80 100644
--- a/src/image.c
+++ b/src/image.c
@@ -3159,16 +3159,18 @@ static bool xpm_load (struct frame *f, struct image *img);
 #define XColor xpm_XColor
 #define XImage xpm_XImage
 #define Display xpm_Display
-#define PIXEL_ALREADY_TYPEDEFED
+#ifdef CYGWIN
+#include "noX/xpm.h"
+#else  /* not CYGWIN */
 #include "X11/xpm.h"
+#endif  /* not CYGWIN */
 #undef FOR_MSW
 #undef XColor
 #undef XImage
 #undef Display
-#undef PIXEL_ALREADY_TYPEDEFED
-#else
+#else  /* not HAVE_NTGUI */
 #include "X11/xpm.h"
-#endif /* HAVE_NTGUI */
+#endif /* not HAVE_NTGUI */
 #endif /* HAVE_XPM */
 
 #if defined (HAVE_XPM) || defined (HAVE_NS)