Cleanup: kmod library detection

Simplify kmod detection code and use a variable to store the detected
lib instead of using the global LIBS variable.

Also remove the --with-kmod-prefix configure option since we don't offer
it for other libs and it's based on user variables which the build
system shouldn't be messing with.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>