deb cleanup

* use installed pg_buildext
* use installed pgxs_debian_control.mk
* don't override 'clean' target in debian/rules
* 'make deb' forces control file regeneration

This reduces crap maintained locally.

It also does mean that to build against server-dev-X.Y package
that is not for Debians default Postgres version, following packages
need to be installed from PGDG (wiki.postgresql.org/wiki/Apt) repo:

  postgresql-client-common
  postgresql-common
  postgresql-server-dev-all