Change the logic in bind.c to only include <malloc.h> if we don't have posix_memalign.

In http://www.open-mpi.org/community/lists/devel/2014/01/13619.php,
Paul Hargrove found a compiler warning on OpenBSD where <malloc.h>
exists, but is not intended to be used (and doesn't error out, so
AC_CHECK_HEADERS says its ok).