GNUmakefile: w/o .built sentry loophole

`make install` in a pristine working copy would go building
everything and then fail because .built is not there.  either
.built cannot list $(artifacts) as its sources, or everything
else needs s/$(artifacts)/all/, which i don't like, because
phony targets break out-of-date evaluation.
GNUmakefile: new targets 'most' and 'html'
fix $(INSTALL_DIR) to not create dirs called "755"
Merge branch 'install_dir-fix'

* install_dir-fix:
  fix $(INSTALL_DIR) to not create dirs called "755"

Conflicts:
	GNUmakefile
specfile: %build phase runs `make check`

supported by a versioned .bs-update-hooks / bsu_update_hook()
running `make wc`