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

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>