Add support for compiling boost.context in iphone

[germandiagogomez]

Hello @danoli3,

Thanks for this great script. It has been helping me a lot so far. However, figuring out how to compile boost.context is being a bit of a hell.

I think it would be a good idea to add support for it directly in the script. There is a stackoverflow question here that gives advice on how it can be achieved:

http://stackoverflow.com/questions/26668858/how-to-build-boost-1-56-0-boostcontext-for-iphone-arm

Thanks for your time and this script. Wihtout it I would not have been able even to use boost in iphone at all probably.