Fix system header problems on Mac OS X

For Mac OS X 10.4, _XOPEN_SOURCE defines _POSIX_C_SOURCE which
hides many symbols from the program.

Breakage noticed and initial analysis provided by Randal
L. Schwartz.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-compat-util.h

@@ -11,8 +11,10 @@
 
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
 
+#ifndef __APPLE_CC__
 #define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */
+#endif
 #define _GNU_SOURCE
 #define _BSD_SOURCE