title: passing compiler flags to boost libraries (such as Thread) which require compilation
body: <p>So I know that the boost libraries are primarily header-only but there are a few which require compilation, for example Boost.Thread.  In Darwin, how do I compile these and pass the -m32 flag so they can be compiled into a 32-bit (i386) binary?  There's this Jamroot thing which I've never heard of and I am not sure at all where to start.</p>
  
  <p>Clarification: I'm not asking how to compile a program with -m32 flag and use the boost libraries.  I'm asking how to compile the Boost libraries themselves with the -m32 flag.</p>