bump required cmake version?

[maddanio]

when building boost i get warnings about too low required cmake version 2.8. Maybe you could bump that to something more recent.

[ttbek]

On older systems it is now cmake 3.2... not even in their package managers. Maybe cget should let us specify which Cmake to use in an easy way... now I'm going to have to monkey around to divine which method is best to tell cget which one to use. And no, in my case it is not appropriate to replace the system-wide Cmake.