Bitrig's standard C++ standard library changed from libstdc++ to libc++.
Also, it uses libc++abi and needs pthread.  While there, fix the libc++
include path. Patch by Patrick Wildt!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207813 91177308-0d34-0410-b5e6-96231b3b80d8