[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.

llvm-svn: 171316
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: bb8cfd0fec3dcd4be6134350ac1da052473266fe
3 files changed