The problem seems to be that configure check (made with C) decides that C11 threads support can be enabled, but those system headers do not work with C++ compiler.
A bit big rework to my taste because of how things depend on each other (must wait until we know what programs we're about to build before can make the threading decision, can't decide AI modules before knowing properties of the threading implementation)
- Removed duplicated "pthreads" result output
Autotools based Qt-client build of all branches fail on Haiku. Meson based builds success.
My Haiku VM has had major update recently - I'm quite sure that it's the reason, and not some freeciv side regression. Regardless, it would be good to get this to work before 3.0.5 release.