macOS: make Homebrew use configurable

macOS: make Homebrew use configurable

On macOS we opportunistically use Homebrew-installed versions of
gettext(3) and msgfmt(1).  Make that behavior configurable by providing
make variables to disable Homebrew usage (NO_HOMEBREW) and to allow
using a non-default installation location (HOMEBREW_PREFIX).

Include and link only the gettext keg via the symlink opt/gettext
pointing to its installed version instead of using the Homebrew prefix.
This is simpler and prevents accidentally including other libraries.

Suggested-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Suggested-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>