Makefile OSX Updates.

Adds onto work by @lennart in https://github.com/openframeworks/openFrameworks/pull/3491.

- Removes 10.6 support.
- Adds 10.10 support.
- Adds configurable `MAC_OS_MIN_VERSION` version, defaults to 10.7 (like apothecary). 

ping @pizthewiz @lennart