OpenBSD added to buildbot

Since we’ve been getting some OpenBSD patches in recent days, I’ve just
added OpenBSD 5.1 (amd64) to our buildbot, so we’ll know going forward
if we’ve broken the build on this OS.

You can see the current build status of this, and all the other targets,

Someone can feel free to send a patch to fix these (possibly)
OpenBSD-specific compiler warnings, too: