When using Clang, pass -stdlib=libstdc++ so we get libstdc++, which has tr1.

This enables building on OS X 10.10 without installing the 10.6 SDK.

(also, I'm able to build now using wxwidgets from Homebrew, making the overall build process much easier.) 

See also (with some info about the -stdlib flag):
http://stackoverflow.com/questions/19774778/when-is-it-necessary-to-use-use-the-flag-stdlib-libstdc