[cmake] Make libcxx standalone compile on Mac OS X.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189738 91177308-0d34-0410-b5e6-96231b3b80d8
[cmake] Provide an option to not install the support headers so that the cmake build can match the make build if requested.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@189739 91177308-0d34-0410-b5e6-96231b3b80d8