Build: Allow building for OSX from Linux

These changes are the small bits needed to allow us to build an OSX dmg from Linux, with no OSX environment needed. They should be completely noninvasive to current builds, but some verification would be much appreciated.

The dependencies are here for now: https://github.com/theuni/osx-cross-depends

While this may not be immediately useful for 0.9.0, it would be nice to have the final release build-able this way, so that we can compare apples/apples when discussing its use in the future.