gitweb/Makefile: add a "NO_GITWEB" parameter

From looking at the {Free,Net,Dragonfly}BSD packages for git[1]
they've been monkeypatching "gitweb" out of the Makefile, let's be
nicer and provide a NO_GITWEB=Y for their use.

For the "all" target this allows for optionally restoring what's been
the status quo before the preceding commit, but now we'll also behave
correctly on the subsequent "make install".

As before our installation of gitweb can be suppressed with
NO_PERL. For backwards compatibility the NO_PERL=Y flag by itself
still doesn't change whether or not we build gitweb, unlike the new
NO_GITWEB=Y flag.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>