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.