Wrap conditionally-compiled C files in #ifdefs

For people editing Tor with an IDE, it can be helpful to have the files which won't get built surrounded with appropriate #ifdef blocks. That way, their IDE won't complain that the file is uncompileable when in fact it's not even supposed to get built.

This is also an issue for people writing their own build scripts/tools for Tor and getting it wrong, but I'm less interested in handling that case.

This ticket is more or less in "Lorax" status. ("Unless someone like you cares a whole awful lot / nothing is going to get better. It's not.") I'll take a clean patch for it in 0.2.5 or later if somebody writes one.