title: makefile source file updates not propagating to target
body: <p>i have this <a href="https://github.com/normalize/www/blob/master/Makefile" rel="nofollow">makefile</a>:</p>
  
  <pre><code>    jade = ./node_modules/.bin/jade
      nlz = ./node_modules/.bin/nlz
  
      build: build/index.js build/index.css build/index.html build/docs.html
  
      build/index.js: $(find client/*.js)
          $(nlz) build client/index.js
  
      build/index.css: $(find client/*.css)
          $(nlz) build client/index.css
  
      build/index.html: pages/home.jade pages/layout.jade
          $(jade) --path pages/home.jade &lt; pages/home.jade &gt; build/index.html
  
      build/docs.html: pages/docs.jade pages/layout.jade
          $(jade) --path pages/docs.jade &lt; pages/docs.jade &gt; build/docs.html
  
      node_modules:
          npm install
  
      clean:
          rm -rf build
  
      .PHONY: clean
  </code></pre>
  
  <p>however, when i update one of the <code>.js</code> or <code>.css</code> files, typing <code>make build</code> gives me <code>make: Nothing to be done for</code>build'.`. what am i doing wrong?</p>