bootstrap: support a prereq of 'rpcgen -' on RHEL5

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.

Signed-off-by: Eric Blake <eblake@redhat.com>