Git.pm: assorted build related fixes.

Git.pm: assorted build related fixes.

 - We passed our own *.a archives as LIBS to the submake that runs
   in perl/; separate LIBS and EXTLIBS and pass the latter which
   tells what the system libraries are used.

 - The quoting of preprocesor symbol definitions passed down to
   perl/ submake was loose and we lost double quotes around
   include directives.  Use *_SQ to quote them properly.

 - The installation location of perl/ submake is not
   architecture neutral anymore, so use SITEARCH instead of
   SITELIB.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>