Bump required CMake version

This bumps the required CMake version to 2.8.6. This allows us to remove some checks/workarounds for old CMake versions. I also took the opportunity to remove various CMake cruft that we never use.

I chose the version number of 2.8.6, since it's new enough that we need it in certain cases, and old enough to be available on current systems (e.g. Ubuntu 12.04 has 2.8.7).

This also establishes a centralized list of minimal dependency versions, with CMake as the sole entry so far.