autoconf: use $(UNITTEST_CXXFLAGS) in tests

As suggested by Roald van Loon
https://github.com/ceph/ceph/pull/538/files#r6260751

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #584 from dachary/master

autoconf: use $(UNITTEST_CXXFLAGS) in tests

Reviewed-by: Sage Weil <sage@inktank.com>