macOS: make sure that gettext is found

macOS: make sure that gettext is found

Due to reasons (some XCode versions seem to include gettext, some
don't?), Homebrew does not expose the libraries and headers in
/usr/local/ by default anymore.

Let's help find them again.

Note: for some reason, this is a change of behavior caused by the
upgrade to Mojave, identified in our Azure Pipeline; it seems that
Homebrew used to add the /usr/local/ directories to the include and link
search path before, but now it no longer does.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>