acinclude.m4: dpdk: link with -ldl if necessary

On some systems libintel_dpdk.a fails to link with libopenvswitch
unless -ldl is used. This should address the issue

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>