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.
Add make check support
Merge branch 'master' of github.com:mato/mill

Conflicts:
	Makefile.am