[plt-scheme] Compile MzScheme to use OS threads

In "Inside PLT MzScheme", it says MzScheme is basically a singlethreaded runtime. But way back in version 103p, it says: "On a fewplatforms, including Windows, Solaris, and Linux, MzScheme can becompiled to map each Scheme thread to a separate operating systemthread. The advantage of the OS-thread configuration is that differentScheme threads can take advantage of different processors on a multi-processor machine." 
 
That's what I want. Has support for this been removed, or is it still possible to compilemzscheme to use OS threads? How? I compiled with pthreads, but thatdidn't appear to work.Thanks.
 
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