This is a stupid question, but I'm new to Ruby, so what's the best way to do this? Would it be to ``cd`` into the ``bison27`` directory and do the usual ``./configure && make install``? But wouldn't that install bison 2.7 when it's not wanted?

Turns out that the reason it's working at the moment is that the Command Line Tools for Xcode provide bison 2.3, but I don't want to rely on that because if Apple ever upgrade that to bison 3 the build will fail.