Makefile: fix NO_PERL bug with gitweb

Makefile: fix NO_PERL bug with gitweb

When the user has defined NO_PERL, we want to skip building
gitweb entirely.  However, the conditional to add
gitweb/gitweb.cgi to OTHER_PROGRAMS was evaluated before we
actually parsed the user's config.mak. This meant that "make
NO_PERL=NoThanks" worked fine, but putting "NO_PERL=NoThanks"
into your config.mak broke the build (it wanted gitweb.cgi
to satisfy "all", but the rule to build it was conditionally
ignored, so it complained).

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