rules:  assume if no config.status, no need to make distclean

A `dpkg-buildpackage` (no `-b`) will run `debian/rules clean`.  This
triggers an `autoreconf` followed by `make distclean`, which leaves
behind cruft such as many generated `GNUmakefile.in` files.  The cruft
raises errors:

    dpkg-source: info: local changes detected, the modified files are:
    [...]
    dpkg-source: info: you can integrate the local changes with
        dpkg-source --commit
    dpkg-source: error: aborting due to unexpected upstream changes [...]
    dpkg-buildpackage: error: dpkg-source -b rtai gave error exit status 2

This patch runs `make distclean` conditionally on `config.status`, an
artifact of `autoreconf`.  If it doesn't exist, assume the tree is
clean and don't run `autoreconf`.
4.0.4:  Update to shabbyx/rtai git rev 5735b81