cmake support for --deps

[kivutar]

I would like to use xgo to compile this project https://github.com/libretro/ludo for Raspberry Pi and maybe later mobile platforms.

There are two C dependencies, both based on cmake:

• https://github.com/kcat/openal-soft
• https://github.com/glfw/glfw

Having cmake support on xgo would help me a lot, as I wasnt' able to automate cross compilation of this project on travis using traditional methods yet.