debian/rules.in:  dpkg-shlibdeps warning exclusions

Add patterns to the `$(SHLIBDEPS_X)` variable that will be prefixed by
`-X` and appended to the `dh_shlibdeps` command.

Remove old, nonsensical `-l` argument (`/usr/lib` is in the default
list already, and `/debian/machinekit/usr/lib` is not a standard path
on any system!)

Conflicts:
	debian/rules.in
configure.in:  silence warning about libxenomai useless dependency

`libxenomai` symbols aren't directly used; silence this warning during
Debian package builds:

    dpkg-shlibdeps: warning: package could avoid a useless dependency
    if debian/machinekit-xenomai/usr/lib/linuxcnc/xenomai/rtapi.so
    debian/machinekit-xenomai/usr/lib/linuxcnc/ulapi-xenomai.so were
    not linked against libxenomai.so.0 (they use none of the library's
    symbols)

The nicer way to do this would be as follows, but `xeno-config` puts
the linker args in `-lnative -lxenomai` order, so `--as-needed` has no
effect:

    flags="$($XENOMAI_THREADS_RTS --ldflags) --as-needed"