Add support for building and testing libc++ without threads to CMake.

Currently hacks must be used in to configure and build libc++ without threads
when using CMake. This patch adds CMake options to enable/disable building with
threads and a monotonic clock.

This patch also propagates the configuration information to lit so the tests
are properly configured as well.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@223591 91177308-0d34-0410-b5e6-96231b3b80d8