doesn't recognize -pthread option

[ygrek]

e.g. https://github.com/ygrek/mldonkey/actions/runs/10475349585/job/29011798015#step:7:26

I am pretty sure I don't need this option for ocaml builds and can find workarounds, but just to let you know

ftr from man gcc:

       -pthread
           Define additional macros required for using the POSIX threads library.  You should use this option consistently for both compilation and
           linking.  This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.