title: Boost-build - dependency on subproject target
body: <p>I have a jamfile-based project where one of the build steps compiles a custom tool (called 'codegen') which I want to use in a later build step. The codegen tool is built in projects/codegen/Jamfile.jam relative to the root, and the executable target is ultimately declared with the line:</p>
  
  <pre><code>install codegen-tool : $(full-exe-target) : &lt;location&gt;$(install-dir) ;
  </code></pre>
  
  <p>In Jamroot.jam, I have the following:</p>
  
  <pre><code>rule codegen ( target : source : properties * )
  {
      COMMAND on $(target) = projects/codegen//codegen-tool ;
      DEPENDS $(target) : projects/codegen//codegen-tool ;
  }
  
  actions codegen bind COMMAND
  {
      $(COMMAND) $(&lt;) $(&gt;)
  }
  
  project.load projects/codegen//codegen-tool ;
  local codegen-input = &lt;blah&gt; ;
  local codegen-output = &lt;blah&gt; ;
  
  make $(codegen-output) : $(codegen-input) : @codegen ;
  alias codegen-output : $(codegen-output) ;
  </code></pre>
  
  <p>When I run the command "b2 codegen-output", I get the error:</p>
  
  <pre><code>don't know how to make project projects/codegen//codegen-tool
  </code></pre>
  
  <p>But running the command "b2 projects/codegen//codegen-tool" is successful. How come I'm not able to reference the codegen-tool target from Jamroot.jam?</p>