If auto-bulding planck, asks to install icu4c

[mfikes]

orion:(master)replete$ script/build
Error: please "brew install icu4c"
orion:(master)replete$ brew list
ant		gmp		libunistring	opencv		sqlite
aurora-cli	icu4c		libxml2		openexr		tbb
awscli		ilmbase		libyaml		openssl		telnet
boot-clj	imagemagick	libzip		openssl@1.1	valgrind
cloc		isl		lumo		pcre		watchman
clojure		jpeg		lz4		pkg-config	wget
cmake		jq		lzlib		planck		x264
curl		lame		maven		python		xvid
eigen		leiningen	mpfr		python@2	xz
ffmpeg		libidn2		mysql@5.7	qt		yarn
freetype	libmpc		node		readline
gcc		libpng		node@8		rlwrap
gdbm		libtiff		numpy		ruby
gettext		libtool		oniguruma	sphinx-doc

Notice that icu4c is installed. If you try to install it, I get:

orion:(master)replete$ brew install icu4c
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 1 tap (homebrew/cask).
==> Migrating cache entries...

Warning: icu4c 62.1 is already installed and up-to-date
To reinstall 62.1, run `brew reinstall icu4c`

[mfikes]

If I do this, things work.

export PKG_CONFIG_PATH="/usr/local/opt/icu4c/lib/pkgconfig"