configure: don't link against libunwind needlessly

Previously we linked against libunwind even if we detected usable
elfutils unwinder. Not a problem for elfutils from mock/koji but can
cause confusion during development.

Closes #162.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Merge pull request #196 from abrt/gh162_configure_dont_link_libunwind

configure: don't link against libunwind needlessly