bootstrap: support a prereq of 'rpcgen -' on RHEL5
On RHEL 5, 'rpcgen --version' spews usage to stderr with status 1,
rather than a version string to stdout with status 0. But libvirt
merely requires a prereq of 'rpcgen -' - that is, it must exist,
but need not have a well-behaved --version.
* build-aux/bootstrap (check_versions): When no specific version
is required, merely check that the app produces an exit status
that indicates its existence.