git-compat-util.h: adjust for SGI IRIX 6.5

Don't define _XOPEN_SOURCE
Do    define _SGI_SOURCE

Defining _XOPEN_SOURCE prevents many of the common functions and macros
from being defined. _Not_ setting _XOPEN_SOURCE, and instead setting
_SGI_SOURCE, provides all of the XPG4, XPG5, BSD, POSIX functions and
declarations, _BUT_ provides a horribly broken snprintf(). SGI does have
a working snprintf(), but it is only provided when _NO_XOPEN5 evaluates
to zero, and this only happens if _XOPEN_SOURCE is defined which, as
mentioned above, prevents many other common functions and defines.

The broken snprintf will be worked around with SNPRINTF_RETURNS_BOGUS in
the Makefile in a later patch.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -52,7 +52,7 @@
 # else
 # define _XOPEN_SOURCE 500
 # endif
-#elif !defined(__APPLE__) && !defined(__FreeBSD__)  && !defined(__USLC__) && !defined(_M_UNIX)
+#elif !defined(__APPLE__) && !defined(__FreeBSD__)  && !defined(__USLC__) && !defined(_M_UNIX) && !defined(sgi)
 #define _XOPEN_SOURCE 600 /* glibc2 and AIX 5.3L need 500, OpenBSD needs 600 for S_ISLNK() */
 #ifndef __sun__
 #define _XOPEN_SOURCE_EXTENDED 1 /* AIX 5.3L needs this */
@@ -62,6 +62,7 @@
 #define _GNU_SOURCE 1
 #define _BSD_SOURCE 1
 #define _NETBSD_SOURCE 1
+#define _SGI_SOURCE 1
 
 #include <unistd.h>
 #include <stdio.h>