Makefile improvements

- Respect system/port flags by default.
- Be verbose and do not hide output. This makes it easier to debug a build.
- Remove the "options" target.
- Remove config.mk: just edit the Makefile or override flags if needed.
- dist: no need to clean before packaging files.
- dist: pipe directly to gzip without an intermediate tarball file.
- Define and add a POSIX marker to the Makefile.