Don't hardcode paths in formulae

This PR includes improvements to the three formulae in this tap to remove hardcoding of paths.

Homebrew already automatically sets up paths in `CFLAGS`, `LDFLAGS`, and `PKG_CONFIG_PATH`; this adjusts the formulae to use those, instead of attempting to override them. (This also has the benefit of letting the formulae work for users with Homebrew installed somewhere other than `/usr/local` or with X11 at a location other than `/opt/X11`.)

This also fixes the formulae on OS X 10.8 and lower, since the system Ruby on those OSs (1.8.x) doesn't support more than one splat (and only at the very end of the list).

In addition, there are a few other miscellanious changes - feel free to ignore those commits if you want.

* Style tweaks (`args << val` with postfix `if`, and `%w[]` arrays for arrays of multiple strings)
* Fix to the `without-av` option description for libtoxcore