[Bug libstdc++/96817] __cxa_guard_acquire unsafe against dynamically loaded pthread

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96817

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Dynamically loading libpthread breaks everything in libstdc++, it just isn't
supported.