Fix checking for C++ compiler

I was unable to get the native extension to build. I'm running Mac OS X 10.9.1 (Mavericks) with LLVM 3.4 (via Homebrew) and Clang 5.0 (from Apple). I can run `clang++ --version` from the command line just fine, but the `find_cxx` check was failing. This patch allows my system to successfully find the compiler and build the native extension.