Mark Makefiles that don't generate files as .PHONY

Mark Makefiles that don't generate files as .PHONY

See
https://www.gnu.org/software/make/manual/html_node/Phony-Targets.html.

These targets aren't the name of a file, they're just a name for some
behaviour to run.

Marking them as .PHONY ensures that even if a file called 'clean' or
'distclean' or 'all' or 'install' exists in the directory, the code will
still be run.