GNUmakefile: default target omits building README.html

it's just the manpage in a different markup.

default target is now "most", which produces just the stuff
necessary for "install" to pass: executable script and
compressed manpage.
specfile: %build phase runs `make most`

as parent says, README.* is just the manpage with a different
markup; it's not that useful in a binary package and the whole
python stack is just deadweight for the build process.
GNUmakefile: directories installed with install -d

... through $(INSTALL_DIR).  i find it better re consistency
and readability.
GNUmakefile: better error message for missing docutils

GNUmakefile:first_in_path calls $(error) if it fails
to find any of the requested commands.