title: Waf: target is directory
body: <p>My wscript file is simple:</p>
  <pre class="lang-py prettyprint-override"><code>def configure(ctx):
      pass
  
  def build(bld):
      bld(rule='mkdir aaa', target='aaa')
      bld(rule='touch bbb', source='aaa', target='bbb')
  </code></pre>
  <p>And first run of <code>waf configure</code> and <code>waf build</code> passes OK. But the second <code>waf build</code> fails with an error:</p>
  <pre><code>source not found: 'aaa' in bld(source='aaa', target=['bbb'], meths=['process_rule', 'process_source'], features=[], path=/home/rnd/prj/prj2/prj-web, idx=2, tg_idx_count=2, rule='touch bbb', posted=True, _name='bbb') in /home/rnd/prj/prj2/prj-web
  </code></pre>
  <p>But if to change the &quot;mkdir&quot; to &quot;touch&quot; - everything works as expected. How to use a directory as a target? (PS. It would be interesting is it possible to use it as a source as well), because it's easy in a plain Makefile, so I had feeling that Waf can handle directories too.</p>