title: Makefile: ignore prerequisite if it does not exist
body: <p>I have a datafile that depends on other datafiles, but the generating script ignores if they are missing. The script automatically creates the makefile, including the ones that do not exist - that in the case they get created, the depending file is recreated.</p>
  
  <pre><code>data: infile1.raw infile2.raw
        touch $@
  infile1.raw:
  infile2.raw:
  </code></pre>
  
  <p>Adding empty rules avoids an error, but triggers an unwanted recreation.</p>
  
  <p>Implicit rules work sometimes, but not reliable - <code>%.raw:</code> causes make to complain that there is no rule to make the <code>infile1.raw</code> targets. Explicitly stating that it does not need to be build with <code>%.raw:;</code> removes the error, but causes again a rebuild. I would hope to avoid wildcards such as</p>
  
  <pre><code>data: $(wildcard infile*.raw)
  </code></pre>
  
  <p>as it very hard to automate this in such a way that not far to many files are matched.</p>
  
  <p>Is there a way to achieve that make ignores missing dependencies?</p>