[PATCH] Do not define _XOPEN_SOURCE on MacOSX as it is too restricting there

[Marco Roeland <marco.roeland@xs4all.nl>]

Defining _XOPEN_SOURCE on Darwin always leads to a restricted set of
available functions and symbols. This can not be cured by adding extra
defines in any way. So there really is only the choice between _not_
defining this symbol on Mac OS X or restricting our usage of functions
and symbols to the POSIX sets that are in term implied by _XOPEN_SOURCE.
The first seems better.

Note the last three lines from this following literal code snippet in
/usr/include/sys/cdefs.h from the Apple Darwin sources:

 * By default newly complied code will actually get the same symbols
 * that the old code did.  Defining any of _APPLE_C_SOURCE, _XOPEN_SOURCE,
 * or _POSIX_C_SOURCE will give you the new symbols.  Defining _XOPEN_SOURCE
 * or _POSIX_C_SOURCE also restricts the avilable symbols to a subset of
 * Apple's APIs.

We want our symbols "avilable" so lets not use _XOPEN_SOURCE on Darwin!

The preferred way of checking specific Apple specific issues is by using
the __APPLE__ predefined macro.

The extra define _XOPEN_SOURCE_EXTENDED does only affect some headers
(like the /usr/include/curses.h header) and can stay.

Patch from Terje Sten Bjerkseth, only added a comment.

diff --git a/git-compat-util.h b/git-compat-util.h
index bc296b3..f056d20 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -11,7 +11,11 @@
 
 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
 
+#if !defined __APPLE__
 #define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
+#else
+			/* On Darwin defining _XOPEN_SOURCE always restricts available functions */
+#endif
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */
 #define _GNU_SOURCE
 #define _BSD_SOURCE