Add support for bootstrapping CMake.

This small buildsystem allows kdesrc-build to bootstrap CMake (from git,
obviously) onto a system that does not have a recent-enough packaged
version of CMake.

The "bootstrap" process is treated as module configuration, so
"configure-flags" is the relevant option (don't worry, those won't be
inherited from global configure-flags). The rest of the build/install
uses make as normal, which is implemented via fallback to
GenericBuildSystem.