[CMake] Fixing the check for Ninja

[CMake] Fixing the check for Ninja

Enabling Ninja Job Pools needs to be dependent on the CMAKE_MAKE_PROGRAM not the CMAKE_GENERATOR. There are generators (like "Sublime Text 2 - Ninja") that also generate ninja build files. Basing of the CMAKE_MAKE_PROGRAM is the best future-proof way to handle this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239076 91177308-0d34-0410-b5e6-96231b3b80d8