macOS/brew: let the build find gettext headers/libraries/msgfmt

macOS/brew: let the build find gettext headers/libraries/msgfmt

Apparently a recent Homebrew update now installs `gettext` into the
subdirectory /usr/local/opt/gettext/[lib/include].

Sometimes the ci job succeeds:
 brew link --force gettext
 Linking /usr/local/Cellar/gettext/0.20.1... 179 symlinks created

And sometimes installing the package "gettext" with force-link fails:
 brew link --force gettext
 Warning: Refusing to link macOS provided/shadowed software: gettext
 If you need to have gettext first in your PATH run:
  echo 'export PATH="/usr/local/opt/gettext/bin:$PATH"' >> ~/.bash_profile

(And the is not the final word either, since macOS itself says:
 The default interactive shell is now zsh.)

Anyway, The latter requires CFLAGS to include /usr/local/opt/gettext/include
and LDFLAGS to include /usr/local/opt/gettext/lib.

Likewise, the `msgfmt` tool is no longer in the `PATH`.

While it is unclear which change is responsible for this breakage (that
most notably only occurs on CI build agents that updated very recently),
https://github.com/Homebrew/homebrew-core/pull/53489 has fixed it.

Nevertheless, let's work around this issue, as there are still quite a
few build agents out there that need some help in this regard: we
explicitly do not call `brew update` in our CI/PR builds anymore.

Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>