Fix comedilib headers

This is a patch from the upstream Debian RTAI packaging, maintained by
Roland Stigge.

It fixes the autoconf m4 macros to look for `comedilib.h` in `/usr/include` rather than `/usr/include/linux`:

    $ find /usr/include -name comedilib.h
    /usr/include/comedilib.h
    $ dpkg-query -S /usr/include/comedilib.h
    libcomedi-dev: /usr/include/comedilib.h
    $ dpkg-query -L libcomedi-dev | grep linux/
    $

After investigation, I found no case where the header's location might
be in `/usr/include/linux/comedilib.h`.  Debian, Ubuntu and Fedora all
install into `/usr/include/comedilib.h`, and building comedilib from
source apparently does, too.  It's starting to look like
`/usr/include/linux/comedilib.h` is an outdated location, or maybe
completely bogus:  this problem has been [reported since at least
2004][1], and hacking around the problem with a symlink is part of [at
least one RTAI tutorial][2]!

This is related to discussion in [my GitHub RTAI package repo][3].

[1]: http://mail.rtai.org/pipermail/rtai/2004-October/009196.html
[2]: http://marc.info/?l=comedi&m=117981204228094
[3]: https://github.com/zultron/rtai-deb/issues/1#issuecomment-57822869
Merge pull request #21 from zultron/comedilib-headers

Fix comedilib headers