joltc failed to start on Windows — "Exception in foreign-procedure: no entry for
pthread_sigmask". concurrency.ss resolves pthread_sigmask/sigemptyset/sigaddset at
load with a top-level (foreign-procedure …), which resolves its symbol eagerly;
those POSIX signal fns don't exist on Windows, so the whole runtime aborted.
Guard the three resolutions (like sched_yield/chmod already are) so a non-POSIX
host yields #f, and make jolt-set-sigint-blocked a no-op when they're unavailable.
The per-thread SIGINT mask is a POSIX-only optimization for the nREPL accept loop;
Windows delivers ^C through the console, and park-until-interrupt still parks on a
condition variable. macOS/Linux resolve the symbols as before — unchanged.
cb03e36088