Fix compilation errors on Mac OS X 10.8.2

By default it was using Apple's clang compiler rather than GCC, and
that compiler didn't like the standard rope implementation
(/usr/include/c++/4.2.1/ext/rope) for whatever reason. I confirmed the
clang compiler couldn't even compile a simple hello-world program that
declared a rope in main(), so there's nothing wrong with the xhp code
in this regard. Clang just doesn't like rope.

Thanks for Andrew Rogers for the fix.