title: How do I perform arithmetic in a makefile?
body: <p>Is it possible to perform some operations on variables in a makefile?
  For instance, defining</p>
  <pre><code>JPI=4
  JPJ=2
  </code></pre>
  <p>is it possible to define in the same makefile a variable <code>JPIJ</code> equal to the expanded value of <code>$(JPI)*$(JPJ)</code>?</p>