[Boost-build] can I determine if a project has been added?

Subject: [Boost-build] can I determine if a project has been added?
From: Spencer E. Olson (olsonse_at_[hidden])
Date: 2010-12-09 14:48:38

Is there some way that one project can test whether another project has been 
added, such that referencing a target in that other project will be resolved?

I'd like to optionally specifiy some metatargets (exe, alias, ...) depending 
on whether a particular project has been added with 'use-project'

Configuration is something like this:
<root-project/Jamroot>
  use-project /lib1 : ./lib1/ ;
  use-project /lib2 : ./lib2/ ;

<lib1/Jamroot>
  lib mylib : ... ;
  ...

<lib2/Jamroot>
  lib mylib : ... ;

  if lib1 {
    lib mylib2 :  ... ;
    exe exe2 : /lib1//mylib ;
  }