[Boost-build] use-project for system and not threads

Subject: [Boost-build] use-project for system and not threads
From: Steve Soule (sts11dbxr_at_[hidden])
Date: 2010-04-24 13:43:01

My project uses the libraries boost_system and boost_thread.  After some
experimentation, I found that the following Jamfile (greatly simplified)
correctly built my executable with the two libraries:

use-project /boost/system : boost/libs/system/build ;

exe myprogram
  : [ glob src/*.cpp ]
    /boost/system//boost_system/<link>static
    /boost/thread//boost_thread/<link>static
  ;

I found that the "use-project" statement was necessary for boost_system
but not boost_thread.  I spent an hour or two looking through the
various Jamfiles, but was unable to figure out why use-project was
necessary for boost_system but not boost_thread.

I'm curious why this would be.  Does anyone know why use-project is
necessary for boost_system but not boost_thread?