Make the Makefile non-mac friendly

Summary:
Add checks in the makefile to not run mac-only things when not on a
mac. (I've only tested on linux, I haven't ensured that this runs mac-only
things on a mac)

Test Plan:
- Run `make`
- Have it not break

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7260