Add a check that the host compiler is modern to CMake, take 1. This is
likely to be reverted and re-applied a few times. The minimum versions
we're aiming at:

  GCC 4.7
  Clang 3.1
  MSVC 17.0 (Visual Studio 2012)

Let me know if something breaks!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199145 91177308-0d34-0410-b5e6-96231b3b80d8