[CMake] Fix c++ abi library configuration on Linux.

You can now configure from the command line using:
-DLIBCXX_CXX_ABI=libsupc++
-DLIBCXX_LIBSUPCXX_INCLUDE_PATHS="path;path

Also documents how to build on Linux.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@171316 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed