[CMake] Add warning for using compile and link pools on unsupported versions...

[CMake] Add warning for using compile and link pools on unsupported versions of CMake or non-ninja generators.

Summary: I've gotten feedback from users on CMake 2.8 that the compile and link pool options were not working. This is expected so I'm adding a warning so we can report invalid configurations to users.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10193

llvm-svn: 239044