[Threads] how to cross-compile boost.threads from cygwin to target win32?

I have been using boost with success since 1 year on the following systems:
1) cygwin built with cygwin gcc/g++,
2) win32 built with cygwin gcc/g++ with -mno-cygwin flag,
3) linux built with both gcc/g++ or intel compiler.

I now want to use boost.threads and I have discovered that the library was not automatically built for
system 2), i.e. win32. How can I force bjam to compile for the target win32 using windows threads
instead of cygwin pthread?
It seems that there are some bjam options like target-os and threadapi but I could not get any documentation
for it. What is the list of possible target-os or threadapi values?

Regards,

F. Bron

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