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>