Installing portaudio on macos

[robbintt]

I had some issues building this because of portaudio on macos today.

It might help uptake to add these steps to the readme, as simply adding portaudio with homebrew is not sufficient, pkg-config is not a dependency and must be installed as well.

https://github.com/RustAudio/rust-portaudio/pull/143/files

[bfops]

That's strange. In the formula it looks like pkg-config is a dependency. It's also listed as one for me:

$ brew info portaudio
portaudio: stable 19.20140130 (bottled), HEAD
Cross-platform library for audio I/O
http://www.portaudio.com
/usr/local/Cellar/portaudio/19.20140130 (32 files, 450.3K) *
  Poured from bottle on 2015-12-25 at 15:21:15
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/portaudio.rb
==> Dependencies
Build: pkg-config ✔
==> Options
--universal
	Build a universal binary
--HEAD
	Install HEAD version

Maybe it's missing in older versions of the formula?