autoconf: make the test for prctl have an effect

We currently test to ensure that prctl() is available, but the results
of that test are a no-op either way. statd calls prctl()
unconditionally, so make configure bail out if prctl() isn't available
since the build will fail in that event anyway.

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