Thanks for the heads-up for Entrian-Free. I’ll fix that ASAP, hopefully later today.
It sounds like the best route is to build on an Ubuntu 16:04 VM, or at least to have one to test on if going down the force_link_glibc_2.23.h route (which would be good because it still uses a more modern compiler).
Does anyone with more experience of this stuff know whether the problems with force_link_glibc_2.23.h that Andrew mentions are always loader errors, or can they be more subtle? If the test of whether force_link_glibc_2.23.h has worked is just “does it load on 16.04?” then that’s easy, but if it actually needs proper testing, for example because something has changed semantically but not in terms of the API/ABI, then that’s much more subtle and would probably make me go down the “build on a 16.04 VM even though you’re using an older compiler” route.