Add -Qunused-arguments for mavericks users.

Summary:
Numpy/etc use flags clang doesn't know about.  This hard-codes in
instructions that used to be on the forge page.

Test Plan: Fingers crossed; I don't use a mac.

Reviewers: johnsullivan, dylan

Reviewed By: dylan

Differential Revision: http://phabricator.khanacademy.org/D9288