Makefile: add a distclean target

i3blocks.1 is removed during a "make clean", which is not convenient
when used from an archive. Create a "distclean" target for this purpose.

Reported-by: Jason Pleau <jason@jpleau.ca>
Makefile: compress dist archive

The generated archive was not gzipped. Add a -z flag to the tar command.

Reported-by: Jason Pleau <jason@jpleau.ca>