We use the library unconditionally, however not check it in
configure phase. Let's produce an error there and not at
compilation phase.
And while at it, drop -ldl from LDFLAGS too. I suspect it's
coming from the same source because in the gbm pkg-config file
private libs are to be linked with dl. But not only we are not a
gbm private library, we don't use dl*() anywhere.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>