configure.ac: Fix help string for --with-statedir= option

The help string for --with-statedir attempts to show "/var/lib/nfs" in
square brackets, but they don't appear on my system (Fedora 13).  Use
the AC_HELP_STRING macro to display the help string properly, like all
the other "with" and "enable" options specified in our configure.ac.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>