Raise the minimum CMake version to 2.8.8 -- we have a report that the

compiler version checking doesn't work on 2.8.7. This feature was
documented in 2.8.10, but existed for an unknown amount of time before
that.

I'm actually happy to revert this and remove the use of the feature if
there is anyone with a specific problem updating CMake. Please just let
me know. I don't want to re-implement this CMake functionality unless
there is a reason, and this is the only real way to find that out.

llvm-svn: 199148