Configure --with-spl{-obj} auto-detect cleanup

Configure --with-spl{-obj} auto-detect cleanup

Because the install location for the spl/zfs-devel headers was
changed we need to refresh the auto-detect code.  Note that
for packaging which already explicitly calls --with-spl{-obj}
nothing has changed.

The updated code is now structured like that in ZFS_AC_KERNEL
and should be cleaner and easier to maintain.  In addition,
it's stricter about detecting a valid source and object
directory.  It requires:

* The source directory contains the file 'spl.release'
* The object directory contains the file 'spl_config.h'
* The following paths will be checked.  Notice the /var/lib/
  and /usr/src paths require that the spl and zfs version be
  matched.  This is done to prevent accidentally mixing releases.

        dnl # 1) /var/lib/dkms/spl/<version>/build
        dnl # 2) /usr/src/spl-<version>/<kernel-version>
        dnl # 3) /usr/src/spl-<version>
        dnl # 4) ../spl
        dnl # 5) /usr/src/kernels/<kernel-version>

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>