Since the addition of the lock manager framework in 6a943419
dlopen is always required, but the checks in configure wasn't changed
to reflect that. This didn't show up directly because the VirtualBox
driver linking dlopen in covered it. But disabling the VirtualBox
driver makes the build fail due to missing dlopen.

Change the dlopen check in configure to pick up dlopen when available.

Reported by Ruben Kerkhof.