[Bug libstdc++/78968] conflict between gnu's __cxa_thread_atexit and LLVM's/FreeBSD's implementation

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78968

--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Trunk no longer defines __cxa_thread_atexit if it's found in libc. We might
want to backport this to the gcc-5-branch and gcc-6-branch.

I will try to test this in a FreeBSD 11 VM some time soon.