title: Makefile with source get error `No such file or directory`
body: <p>I have a really simple <code>Makefile</code> that's doing <code>source</code> to set ENV variables. But it doesn't work if I call it from <code>Makefile</code></p>
  
  <p>I get this error</p>
  
  <pre><code>make dev
  source ./bin/authenticate.sh
  make: source: No such file or directory
  make: *** [dev] Error 1
  </code></pre>
  
  <p>The script exists. </p>
  
  <p>If I run this in command-line it works. </p>
  
  <pre><code>source ./bin/authenticate.sh
  Works!
  </code></pre>
  
  <p>This is my <code>Makefile</code></p>
  
  <pre><code>test:
      pytest -s
  
  dev:
      source ./bin/authenticate.sh
  </code></pre>
  
  <p>I'm using OSX. I'm not sure if this would make a difference.</p>