[boost] [Thread] What is the meaning of BOOST_THREAD_USE_LIB?

Hi, I always use boost as static libraries. For a long time, I have been used to define macro BOOST_THREAD_USE_LIB on windows when cross-compiling from linux to windows with mingw. I have now made a new cross-compiler conformant with pthread (the previous one had only win32 api) but then boost does not build (with threadapi=posix). It seems that not defining BOOST_THREAD_USE_LIB helps. I do not remember why I used to define BOOST_THREAD_USE_LIB a long time ago and cannot find any help in the doc (http://www.boost.org/doc/libs/1_64_0/doc/html/thread/build.html). What is the meaning of BOOST_THREAD_USE_LIB? Frédéric