Makefile: allow building without perl

For systems with a missing or broken perl, it is nicer to
explicitly say "we don't want perl" because:

  1. The Makefile knows not to bother with Perl-ish things
     like Git.pm.

  2. We can print a more user-friendly error message
     than "foo is not a git command" or whatever the broken
     perl might barf

  3. Test scripts that require perl can mark themselves and
     such and be skipped

This patch implements parts (1) and (2). The perl/
subdirectory is skipped entirely, gitweb is not built, and
any git commands which rely on perl will print a
human-readable message and exit with an error code.

This patch is based on one from Robin H. Johnson.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unimplemented.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+echo >&2 "fatal: git was built without support for `basename $0` (@@REASON@@)."
+exit 128