Thank you, Luke, this settles the situation then. It is ironic that I moved to Arch, some years ago, exactly because the proprietary drivers for my old card were dropped from Ubuntu and the rest, the only way to get Houdini running with it was to install the driver from the AUR.
Well, for anyone trying to install the drivers on Arch, the situation is hopeless. The 4.9-kallsyms kernel builds properly, but mesa won't. It needs to link against a particular version of llvm, but is unable to build with it, so simply downgrading it won't help. To compound the problem, the last update to glibc breaks everything, and downgrading it requires downgrading gcc and gcc-libs which can pretty much break the whole system. The AUR maintainers of the driver and the patched mesa seem unresponsive, which can be expected since the community is satisfied with the open source driver performance (for games).
I migrated the system to Ubuntu 18.04, and found the situation even worse. With the proprietary drivers, I'm not even able to login.
I'm at a loss.
Thanks for any assistance.
Edit: It seems the proprietary driver only works with Ubuntu 16.04. I think I will return the board, get a nvidia and go back to Arch.