Allow PERL_PATH="/usr/bin/env perl"

Allow PERL_PATH="/usr/bin/env perl"

There is a mechanism PERL_PATH in the Makefile to specify path to
Perl binary, but sometimes it is convenient to let 'env' figure
out where Perl comes from, with PERL_PATH="/usr/bin/env perl".

Allowing this would make things easier to MacPorts, where we wish
to work with the MacPorts perl if it is installed, but fall back
to the system perl if it isn't.

Signed-off-by: Bryan Larsen <bryan@larsen.st>
Signed-off-by: Junio C Hamano <junkio@cox.net>