help: report on whether or not gettext is enabled

help: report on whether or not gettext is enabled

When users report that Git has no localized output, we need to check not
only their locale settings, but also whether Git was built with GETTEXT
support in the first place.

Expose this information via the existing build info output by adding a
"gettext: enabled" line to `git version --build-options` (and therefore
also to `git bugreport`) when `NO_GETTEXT` is not defined at build time.

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>