meson: skip gitweb build when Perl is disabled

meson: skip gitweb build when Perl is disabled

It is possible to configure a Git build without Perl when disabling both
our test suite and all Perl-based features. In Meson, this can be
achieved with `meson setup -Dperl=disabled -Dtests=false`.

It was reported by a user that this breaks the Meson build because
gitweb gets built even if Perl was not discovered in such a build:

    $ meson setup .. -Dtests=false -Dperl=disabled
    ...
    ../gitweb/meson.build:2:43: ERROR: Unable to get the path of a not-found external program

Fix this issue by introducing a new feature-option that allows the user
to configure whether or not to build Gitweb. The feature is set to
'auto' by default and will be disabled automatically in case Perl was
not found on the system.

Reported-by: Daniel Engberg <daniel.engberg.lists@pyret.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>