acinclude: Use AC_SEARCH_LIBS for linking with dl.

acinclude: Use AC_SEARCH_LIBS for linking with dl.

DPDK uses dlopen to load plugins and we need to search for
library containing this function. But we should not do this
in a loop because 'AC_SEARCH_LIBS' could do this for us.
Also, 'AC_SEARCH_LIBS' prints user-visible messages that are
useful for debuging.
Also added the new 'checking' message and code normalized to
be more readable.

With this change we'll have following additional messages:

  checking for library containing dlopen... -ldl
  checking whether linking with dpdk works... yes

Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>