Be more explicit for FreeBSD's new linker.

On recent FreeBSD-CURRENT, the linker behaves differently w.r.t. indirect library dependencies. We can make it happy again by explicitly specifying that we depend on libthr and, in the libc++ case, libcxxrt.
The tool is now 'tesla print', not 'tesla graph'.

It seems that this demo was still working in the Jenkins setup because the old tesla-graph binary wasn't deleted after we renamed it to tesla-print.

A better build system might've deleted this for us. :)
Update the 'tesla' script with graph->print, too.