title: howto call makefile in sub-directory
body: <p>I have a developing project, and use another lib project source code, and it has its own makefile to produce the lib file. how to write my project makefile to call another makefile?</p>
  
  <pre><code>myproject/
    -makefile 
    src/
  
    threePartyLibProject/
           -makefile
           src/
           inc/
  </code></pre>
  
  <p>how to writing my project makefile to call the threePartyLibProject makefile, and how link to its produced libs?</p>
  
  <p>very highly appreciated your help,</p>
  
  <p>thanks very much.</p>