Update Makefile to reflect that TestNumThreads now uses std::thread instead of pthreads.
Update Makefile to reflect that TestNumThreads now uses std::thread instead of pthreads.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@237919 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb