BOOST_THREAD_USES_CHRONO not consistent on build and use

As reported in https://github.com/boostorg/boost_install/issues/27, the following program:
```
#include

int main() {
boost::condition_variable cv;
boost::mutex mutex;
boost::unique_lock lock(mutex);
cv.wait_for(lock, boost::chrono::seconds(1));
return 0;
}
```
compiles on Linux but doesn't link without specifying `-lboost_chrono`, even though the Boost.Thread build/Jamfile doesn't specify Chrono as a dependency on Pthread platforms. The reason is that the Jamfile defines `BOOST_THREAD_DONT_USE_CHRONO` on Pthread platforms when Boost.Thread is built. However, when Boost.Thread is used by user code, `BOOST_THREAD_DONT_USE_CHRONO` is not defined and https://github.com/boostorg/thread/blob/develop/include/boost/thread/detail/config.hpp#L143 defines `BOOST_THREAD_USES_CHRONO`. This enables the `wait_for` overload and allows the code above to compile, even though the compiled library wasn't built with `BOOST_THREAD_USES_CHRONO`.

I'm not sure whether this behavior can be considered correct. One may argue that `BOOST_THREAD_USES_CHRONO` should not be defined on Pthread platforms by default, to match the settings with which the library is built.