Add IDL compiler option to specify the output directory for the generated code (requires cyclonedds PR #922)

[obiltschnig]

This adds support for idl_pstate->outdir to idlcxx.

Requires eclipse-cyclonedds/cyclonedds#922

[ahoenerBE]

@obiltschnig 922 was closed, check out 1263
Should make life pretty easy to add the new features, I hope

[eboasson]

We really should do something about this: it is not right that we handle the output directory option only for C. @thijsmie or @poetinger, perhaps you have a label for things that really ought to be done in the short term?

(Judging by the original suggestion and the work from @ahoenerBE in cyclonedds, I am pretty sure it should be straightforward enough to just do it — so that, too, is an option.)