Git.pm: Support for perl/ being built by a different compiler

Git.pm: Support for perl/ being built by a different compiler

dst_ on #git reported that on Solaris 9, Perl was built by Sun CC
and perl/ is therefore being built with it as well, while the rest
of Git is built with gcc. The problem (the first one visible, anyway)
is that we passed perl/ even various gcc-specific options. This
separates those to a special variable.

This is not really meant for an application yet since it's not clear
if it will alone help anything.

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