Marc says build is
./configure --enable-multiple-threaded-vms --enable-smp
make; make bootstrap; make bootclean; make
and "the bootstrap step is needed because the Scheme code of the thread
scheduler needs to be recompiled".
Related gambit/include.h DEFINE:s are ___MAX_PROCESSORS, which is 64, and
___MULTIPLE_THREADED_VMS which is set.
thread-send/receive currently crashes, and otherwise looks perfect.