rebuild requires libgc and pthreads. libgc is portable and available at:
http://www.hpl.hp.com/personal/Hans_Boehm/gc/

pthreads should be a component of all UNIX-alikes. A version of pthreads for
Windows exists:
http://sourceware.org/pthreads-win32/

On Windows you will need to specify the library name of pthreads-win32, e.g.

make THREADS=-lpthread