Make build more parallelizable.
Make build more parallelizable.

It looks like it's possible to build the custom Xamarin targets
(monotouch/monodroid/xamarinmacmobile/xamarinmacfull) in parallel,
so modify the Makefile to support that.

This makes a joint monotouch+xamarinmac build ~40s faster
(from 2:30 to 1:50 on my machine).

7sharp9/fsharp,7sharp9/fsharp