ensure that we can compile gc with pthreads enabled and link and run

implement primitive support for threads

idea: could use openmp (supported by gcc pragmas) with libgc, if we do this:

    You would have to explicitly register any OpenMP threads.  With the latest GC
    versions, that's possible on some platforms with GC_register_my_thread. Having
    the linker intercept the pthread_create call may be another option.  Either way
    this definitely requires care.

    Hans
