use the cmake package to cross compile packages

I don't know why there's been a cmake package in mxe. 
But we could use it to build cmake-based packages within mxe.

This would simplify static builds of cmake-based packages, because we could track lib-dependencies as patches for CMake. See https://lists.gnu.org/archive/html/mingw-cross-env-list/2011-11/msg00011.html and #514 .