Implement autoconf build system

- Checks for dependencies (Ruby, Ribosome, libuv)
- Generates all files (mill, stdmill) from source
- Installs mill (command) and stdmill (library, include files)
Separate code generation phase from end-user use

- Mill source itself (.dna, clexer) is moved to gen/
- configure gets an option --enable-gen, which checks for dependencies
  and allows regeneration of MIll itself from files in gen.
- end-user dependencies are thus only Ruby and libuv.