Apparently, libintl is part of the gettext package. I did the following, possibly redundant reinstall to make sure my copy was up-to-date:
$ brew install gettext
Warning: gettext-0.18.3.2 already installed
$ brew reinstall gettext
==> Reinstalling gettext
==> Downloading http://ftpmirror.gnu.org/gettext/gettext-0.18.3.2.tar.gz
Already downloaded: /Library/Caches/Homebrew/gettext-0.18.3.2.tar.gz
==> ./configure --prefix=/usr/local/Cellar/gettext/0.18.3.2 --with-included-gettext --with-included-glib --with-included-libcroco --with-included-libunistring --with-emac
==> make
==> make install
==> Caveats
This formula is keg-only, so it was not symlinked into /usr/local.
OS X provides the BSD gettext library and some software gets confused if both are in the library path.
Generally there are no consequences of this for you. If you build your
own software and it requires this formula, you'll need to add to your
build variables:
LDFLAGS: -L/usr/local/opt/gettext/lib
CPPFLAGS: -I/usr/local/opt/gettext/include
It says in the above output that brew doesn't symlink the library, which might explain why install.packages can't find it. What did the trick was adding a library path into ~/.R/Makevars like so:
PKG_LIBS=-L/usr/local/Cellar/gettext/0.18.3.2/lib