Some changes for separate Linux source and build trees.

Some changes for separate Linux source and build trees.
Add LINUX_SRC_DIR variable to point to Linux source tree (LINUX_DIR points
to the Linux build tree).  It can be set with a new --with-linuxsrcdir
option or determined automatically from the build tree.
Changed some configure tests to check the Linux source tree instead of the
build tree.  The affected tests are COMEDI_CHECK_PCMCIA_PROBE,
COMEDI_CHECK_LINUX_KBUILD and COMEDI_CHECK_HAVE_MUTEX_H.  The
COMEDI_CHECK_LINUX_KBUILD test has been simplified because it no longer
has to check for separate source and build trees.