Should be able to
change the build script to use a static libstdc++. It'll be a bit bigger (for TF2, the compiled extension ends up being 1.5MB compared to ~0.4), but it shouldn't depend on any external libraries at that point.
The extension could also maybe be built with
AMTL instead to avoid standard library dependencies; most things that are in use seem to have equivalents there.
(I'd look into it myself but I don't have a SSH key to Github set up on my build environment as it's technically borrowed, CBA to move stuff between that and my Windows box.)
That said, here's the extensions compiled with static libstdc++ (Ubuntu 16.04.3 LTS).