Modernize Autotools usage, real instructions

The code has bitrotted somewhat since the last commit, and
frankly, Autotools has never been a friendly technology.
But this should clean up the way it handles on modern
systems, and give users a bit more guidance in setting up
this synth.

That's not to say it will be a guaranteed smooth ride, but
it never hurts to do a bit of dusting and sand the
sharpness off some edges.
Convert README to Markdown version (README.md)

This way it's still perfectly readable as plaintext, but
also conveys formatting benefits to users on Github.