rtapi/Submakefile: force link if file exists
configure.in:  fix up detection of rtai-config and kbuild kernel dirs

- fix incorrect logic in rtai-config detection
  - --with-rtai-config never worked!  oops
  - --without-rtai, errors later on when this is ignored
    - '$RTAI_KERNEL_THREADS_RTS --prefix' used later
    - kernel detection ran, ignoring $with_rtai_kernel=no
  - all logic paths tested for rtai-config detection, e.g.
    --with-rtai-config=bogus, --with-rtai-kernel but rtai-config absent, etc.

FIXME  Strange problem: Running

  ./configure --without-xenomai-kernel --without-rtai-kernel

generates seemingly unrelated errors later:

  checking X11/extensions/Xinerama.h usability... yes
  checking X11/extensions/Xinerama.h presence... no
  configure: WARNING: X11/extensions/Xinerama.h: accepted by the compiler,
    rejected by the preprocessor!
  configure: WARNING: X11/extensions/Xinerama.h: proceeding with the
    compiler's result

When autoconf checks the kernel version.h, it checks several more
headers than when not; presumably these are somehow persistent and
affect the later cpp run.