v0.18.1 - Opensuse, a #cmd_dir fix, OS X build tools.

* Basic opensuse/zypper support thanks to Robert Warmbier.
* A fix for #cmd_dir; directories were recognised as commands when their
  names matched. Thanks to Chris Heisterkamp.
* The 'xcode build tools' dep now tolerates gcc being installed in a
  separate location to the rest of the toolchain, to account for the
  case where it's installed separately via homebrew.