Add MSVC to Makefile

Add MSVC to Makefile

Enable MSVC builds with GNU Make by simply calling
    make MSVC=1
(Debug build possible by adding DEBUG=1 as well)

Two scripts, clink.pl and lib.pl, are used to convert certain GCC
specific command line options into something MSVC understands.
By building for MSVC with GNU Make, we can ensure that the MSVC
port always follows the latest code, and does not lag behind due
to unmaintained NMake Makefile or IDE projects.

Signed-off-by: Marius Storm-Olsen <mstormo@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>