git-compat-util.h: tweak the way _XOPEN_SOURCE is set on Solaris

git-compat-util.h: tweak the way _XOPEN_SOURCE is set on Solaris

On Solaris, when _XOPEN_EXTENDED is set, its header file forces the
programs to be XPG4v2, defeating any _XOPEN_SOURCE setting to say we are
XPG5 or XPG6.  Also on Solaris, XPG6 programs must be compiled with a c99
compiler, while non XPG6 programs must be compiled with a pre-c99 compiler.

So when compiling on Solaris, always refrain from setting _XOPEN_EXTENDED,
and then set _XOPEN_SOURCE to 600 or 500 based on whether a c99 compiler
is being used or not.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>