Hi Eric, thank you for the patch. We have also noticed that the builds are slow, when we set up out test server (ci.biolab.si). We solved this by using ccache (http://ccache.samba.org/), which is suggested in the same StackOverflow answer.
The monkeypatch is recommended "Since you have a single extension, ... (they are independent)". Orange builds multiple extensions (orange, orangene, orangeom, statc, corn), with dependencies among them. The parallelisation code looks quite simple, and does not care about the possible dependencies, which might result in random build failures, if the files would be built in the wrong order.
I might be wrong and am willing to discuss this, but at the moment I am more inclined towards declining this pull request.