meson: enable auto-discovered "gitweb"

meson: enable auto-discovered "gitweb"

In 7d549fe317 (meson: skip gitweb build when Perl is disabled,
2024-12-20) we have started to conditionally enable "gitweb" based on
whether or not Perl is enabled. By accident though that change causes us
to not build gitweb in case its feature flag is set to "auto" even if
autoconfiguration determines that it could be built. This is because we
use "gitweb_option.enabled()", which only checks whether the feature has
been explicitly enabled.

Fix the issue by using `gitweb_option.allowed()` instead, which returns
true in case it is either explicitly enabled or set to "auto". This also
works for the case where the feature becomes auto-disabled due to Perl
not being present because we use `disable_auto_if(not perl.found())`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>