Create a simple makefile and allow regressions to use it.

[threeifbyair]

One of the frustrating things about cppfront is that I always have to remember the precise incantation to build cppfront. In addition, the regression tests always build cppfront whether or not it needs to be built.

This PR adds a simple Makefile. Nothing special and explicitly marked as optional. In addition, it allows the regression tests to be run with a pre-built version of cppfront. This allows us (for instance) to build a sanitized cppfront and run it through the regressions to see if we get any memory or undefined behavior errors.