Remove USE_MACPORTS option since Homebrew’s wxmac is now multilib build again.
Merge branch 'rust-servo': Remove USE_MACPORTS option since Homebrew’s wxmac is now multilib build again.
Make wxRust build able on OS X 10.9 Marvericks.

 * Teach bindgen the path to libclang.dylib by specifying -L rustc flag and DYLD_FALLBACK_LIBRARY_PATH environment variable.
Merge branch 'rust-servo': Make wxRust build able on OS X 10.9 Marvericks.