The issue, to which I have not yet been able to make a minimal testcase,
involves mixing g++-7 binaries with g++-8 generated libraries, both using
std::make_shared in some fashion.

Just by looking at the GCC commits, I found something that looks like a bugfix
in gcc master:

commit 3c1c2a8c6bc4a24ebd235a3c2ba8e6b89fbd3208
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Fri Jan 18 21:28:48 2019 +0000

    PR libstdc++/88782 avoid ODR problems in std::make_shared
    ...
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@268086
138bc75d-0d04-0410-961f-82ee72b054a4

I believe it is that bug that I am hitting with openSUSE's gcc 8.