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.

2 files changed, 10 insertions, 13 deletions

diff --git a/configure.ac b/configure.ac
index 069a7260e88..2674806cadc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3245,34 +3245,29 @@ fi
 
 ### Use -lXpm if available, unless '--with-xpm=no'.
 ### mingw32 doesn't use -lXpm, since it loads the library dynamically.
-### In the Cygwin-w32 build, we need to use /usr/include/noX/X11/xpm.h
-### rather than /usr/include/X11/xpm.h, so we set CPPFLAGS (and
-### LDFLAGS) accordingly.
+### The Cygwin-w32 build uses <noX/xpm.h> instead of <X11/xpm.h>, so
+### we need to set LDFLAGS accordingly.
 HAVE_XPM=no
 LIBXPM=
 if test "${HAVE_W32}" = "yes" && test "${opsys}" = "cygwin"; then
   if test "${with_xpm}" != "no"; then
-    SAVE_CPPFLAGS="$CPPFLAGS"
     SAVE_LDFLAGS="$LDFLAGS"
-    CPPFLAGS="$CPPFLAGS -I/usr/include/noX"
     LDFLAGS="$LDFLAGS -L/usr/lib/noX"
-    AC_CHECK_HEADER(X11/xpm.h,
+    AC_CHECK_HEADER(noX/xpm.h,
       [AC_CHECK_LIB(Xpm, XpmReadFileToImage, HAVE_XPM=yes)])
     if test "${HAVE_XPM}" = "yes"; then
       AC_MSG_CHECKING(for XpmReturnAllocPixels preprocessor define)
       AC_EGREP_CPP(no_return_alloc_pixels,
-      [#include "X11/xpm.h"
+      [#include "noX/xpm.h"
 #ifndef XpmReturnAllocPixels
 no_return_alloc_pixels
 #endif
       ], HAVE_XPM=no, HAVE_XPM=yes)
 
       if test "${HAVE_XPM}" = "yes"; then
-        REAL_CPPFLAGS="$REAL_CPPFLAGS -I/usr/include/noX"
         AC_MSG_RESULT(yes)
       else
         AC_MSG_RESULT(no)
-        CPPFLAGS="$SAVE_CPPFLAGS"
         LDFLAGS="$SAVE_LDFLAGS"
       fi
     fi
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)