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>