Update OS X toolchain

Port the changes in #18331 (moved) to rbm

Started on this in, https://github.com/TheTorProject/tor-messenger-build/commit/65d9353d2e083ec2aef4fe4616a3c80d4390dc68

but boklm agree to takeover.

(At the same time, it might be worth switching the Windows distribution to Debian-7.11 so that all platforms are building on it.)