Patch from upstream darcs repository, see this bugzilla entry for details:
http://bugs.ghostscript.com/show_bug.cgi?id=690932

Wed Jun 16 10:29:54 UTC 2010  tor@ghostscript.com
  * Do runtime endianness test instead of relying on unreliable preprocessor macros.
diff -rN -u old-mupdf/apps/x11_image.c new-mupdf-1/apps/x11_image.c
--- old-mupdf/apps/x11_image.c	2010-06-16 12:33:35.000000000 +0000
+++ new-mupdf-1/apps/x11_image.c	2010-06-16 12:33:35.000000000 +0000
@@ -10,7 +10,7 @@
 # define _XOPEN_SOURCE 1
 #endif
 
-#include <fitz.h>
+#include "fitz.h"
 
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
@@ -205,11 +205,10 @@
 	unsigned long rs, gs, bs;
 
 	byteorder = ImageByteOrder(info.display);
-#if BYTE_ORDER == BIG_ENDIAN
-	byterev = byteorder != MSBFirst;
-#else
-	byterev = byteorder != LSBFirst;
-#endif
+	if (fz_isbigendian())
+		byterev = byteorder != MSBFirst;
+	else
+		byterev = byteorder != LSBFirst;
 
 	rm = info.visual.red_mask;
 	gm = info.visual.green_mask;
diff -rN -u old-mupdf/draw/imageunpack.c new-mupdf-1/draw/imageunpack.c
--- old-mupdf/draw/imageunpack.c	2010-06-16 12:33:35.000000000 +0000
+++ new-mupdf-1/draw/imageunpack.c	2010-06-16 12:33:36.000000000 +0000
@@ -31,6 +31,12 @@
 		justinvert &= min[i] == 255 && max[i] == 0 && sub[i] == -255;
 	}
 
+	unsigned mask;
+	if (fz_isbigendian())
+		mask = 0x00ff00ff;
+	else
+		mask = 0xff00ff00;
+
 	if (!needed)
 		return;
 
@@ -51,11 +57,7 @@
 				wh = wh - 2 * hwh;
 				while(hwh--) {
 					unsigned in = *wp;
-#if BYTE_ORDER == LITTLE_ENDIAN
-					unsigned out = in ^ 0xff00ff00;
-#else
-					unsigned out = in ^ 0x00ff00ff;
-#endif
+					unsigned out = in ^ mask;
 					*wp++ = out;
 				}
 				p = (byte *)wp;
diff -rN -u old-mupdf/fitz/base_cpudep.c new-mupdf-1/fitz/base_cpudep.c
--- old-mupdf/fitz/base_cpudep.c	2010-06-16 12:33:35.000000000 +0000
+++ new-mupdf-1/fitz/base_cpudep.c	2010-06-16 12:33:36.000000000 +0000
@@ -10,6 +10,12 @@
 /* global run-time constant */
 unsigned fz_cpuflags = 0;
 
+int fz_isbigendian(void)
+{
+	static const int one = 1;
+	return *(char*)&one == 0;
+}
+
 #ifndef HAVE_CPUDEP
 
 void fz_cpudetect(void)
diff -rN -u old-mupdf/fitz/fitz_base.h new-mupdf-1/fitz/fitz_base.h
--- old-mupdf/fitz/fitz_base.h	2010-06-16 12:33:35.000000000 +0000
+++ new-mupdf-1/fitz/fitz_base.h	2010-06-16 12:33:36.000000000 +0000
@@ -86,6 +86,8 @@
 /* treat as constant! */
 extern unsigned fz_cpuflags;
 
+int fz_isbigendian(void);
+
 /*
  * Base Fitz runtime.
  */