Makefile improvements

- PREFIX used instead of hardcoded /usr
- CC set only if environment variable is not
- A few mkdirs added
- Warning flags removed... if you don't have them in your CFLAGS, then you don't need to see them
- conversion scripts weren't been copied from the subfolder; fixed.
- uninstall now *does not* remove the user's config file

Signed-off-by: Steven Honeyman <stevenhoneyman@users.noreply.github.com>