This patch adds support to compile and run git on 12 additional platforms.
The platforms are based on UNIX Systems Labs (USL)/Novell/SYS V code base.
The most common are Novell UnixWare 2.X.X, SCO UnixWare 7.X.X,
OpenServer 5.0.X, OpenServer 6.0.X, and SCO pre OSR 5 platforms.

Looking at the the various platform headers, I find:

	#if defined(_KERNEL) || !defined(_POSIX_SOURCE) \
	     && !defined(_POSIX_C_SOURCE) && !defined(_XOPEN_SOURCE)

which hides u_short and other typedefs that other header files on these
platforms depend on.  WIth _XOPEN_SOURCE defined, sources that include
system header files that depend on the typedefs such as u_short cannot be
compiled on these platforms.

__USLC__ indicates UNIX System Labs Corperation (USLC), or a Novell-derived
compiler and/or some SysV based OS's.

__M_UNIX indicates XENIX/SCO UNIX/OpenServer 5.0.7 and prior releases
of the SCO OS's.  It is used just like Apple and BSD, both of these
shouldn't have _XOPEN_SOURCE defined.

This is with suggestions and modifications from

Daniel Barkalow, Junio C Hamano, Thomas Harning, and Jeremy Maitin-Shepard.

Signed-off-by: Boyd Lynn Gerber <gerberb@zenez.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>