fix X_DISPLAY_MISSING redefined warnings when X is disabled

This is set up in config.h by configure, so avoid defining it again.

src/bin/imlib2_conv.c

@@ -8,7 +8,9 @@
 #include <stdio.h>
 #include <string.h>
 
+#ifndef X_DISPLAY_MISSING
 #define X_DISPLAY_MISSING
+#endif
 #include <Imlib2.h>
 
 #define PROG_NAME "imlib2_conv"

src/lib/api.c

@@ -10,8 +10,10 @@
 #include <X11/Xutil.h>
 #include <X11/extensions/shape.h>
 #else
+#ifndef X_DISPLAY_MISSING
 #define X_DISPLAY_MISSING
 #endif
+#endif
 
 #include "Imlib2.h"
 #include "blend.h"

src/lib/image.h

@@ -5,8 +5,10 @@
 #ifdef BUILD_X11
 #include <X11/Xlib.h>
 #else
+#ifndef X_DISPLAY_MISSING
 #define X_DISPLAY_MISSING
 #endif
+#endif
 
 #include <Imlib2.h>
 

src/lib/script.c

@@ -13,8 +13,10 @@
 #ifdef BUILD_X11
 #include <X11/Xlib.h>
 #else
+#ifndef X_DISPLAY_MISSING
 #define X_DISPLAY_MISSING
 #endif
+#endif
 
 #include "dynamic_filters.h"
 #include "image.h"