xlc 13.1.0 crashes because of our may_alias attributes in nolibxml.c
on Power7. libxml.c and nolibxml.c are the only may_alias users for now,
so change our configure check to match the actual code using it.
Thanks to Paul Hargrove for reporting and debugging the issue,
and providing the patch.
https://www.open-mpi.org/community/lists/devel/2016/05/18918.php
(cherry picked from open-mpi/hwloc@0ab7af5e90)