t/gitweb-lib.sh: skip gitweb tests when perl dependencies are not met

t/gitweb-lib.sh: skip gitweb tests when perl dependencies are not met

Linus noticed that we go ahead testing gitweb and fail miserably on a
box with Perl but not perl-CGI library. We already have a code to detect
lack of Perl and refrain from testing gitweb in t/gitweb-lib.sh (by the
way, shouldn't it be called t/lib-gitweb.sh?), so let's extend it
to cover this case as well.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>