Fix the check for Ninja in the CMake build.

Fix the check for Ninja in the CMake build.

The current check never passes, because CMAKE_MAKE_PROGRAM, at least on Linux,
includes the full path to the "ninja" binary; this effectively disables
compile/link jobs pools.

Use CMAKE_GENERATOR STREQUAL "Ninja" as a more reliable check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239069 91177308-0d34-0410-b5e6-96231b3b80d8