configure: set the foreign flag

This is required since we do not bundle the necessary files (NEWS,
INSTALL et al.) Setting it in configure.in is needed to be able
to run autoreconf (in lieu of the weird autogen.sh), with just:

    autoreconf -f -i

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
autogen: cleanup, now that we can run autoreconf

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
configure.in: rename to configure.ac

The correct name is configure.ac, see:
    https://www.gnu.org/software/autoconf/manual/autoconf.html#Writing-Autoconf-Input

    ---
    Previous versions of Autoconf promoted the name configure.in,
    which is somewhat ambiguous [...]. Using configure.ac is now
    preferred.
    ---

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Merge pull request #130 from yann-morin-1998/yem/fixes

yem/fixes: cleanups in configure.in