Some monkeying around with 'zypper packages --installed-only' and sed and friends later, I tested 'osc build' with a very long list of BuildRequires that matched the list of all installed packages on a machine where rpmbuild succeeds. In conclusion it looks like *something* in the environment of 'osc build' and a OBS builder makes makensis very unhappy. This is quite bad for Xiph.org as it means I can't have automated builds of installers on OBS.