Make Xplanet an optional runtime-only dependency

So far, the code for Xplanet was built only if the xplanet tool was
found at build time, which was not really needed since all it changed
was a "build or not build" setting. Also, since it is a tool that can
be present or missing at runtime, doing the checks at that point is
still needed.

Hence, making the Xplanet code unconditionally built: the amount of code
is small, and this reduces the risk of bit rotting. The cmake check is
left to give an indication about the runtime dependency, but that is
about it.

Furthermore, add a existance check when xplanet is invoked, so there is
a proper message about its lack.