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phony targets break out-of-date evaluation.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
WIP: breaking change in bsu_tarball_hook behavior

bs-update cannot repack all tarballs, repacking breaks
checksums/signatures.  this feature is in fact an idiosyncracy:
i wrote it for the special case of the packager being the
upstream maintainer as well.

bsu_tarball_hook() now must repack the tarball on its own: it's
run in a directory containing the tarball, with its name in $1.
possible implementation looks very much like the late repack():

  bsu_tarball_hook()
  (
    set -e
    local tarball=${1?}
    local dir=${tarball%.tar.gz}
    mkdir $dir
    tar -xzf $tarball --strip-components=1 -C $dir
    cd $dir
    do_stuff
    tar -czf $tarball $dir
    rm -rf $dir
  )