Port to 12 other Platforms.

Port to 12 other Platforms.

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>