title: Pass variable to child make only if defined
body: <p>I can pass a variable to child make using <code>make -C VAR=$(VAR) target</code> (preferring this to <code>export</code> because the variable is only meaningful for a single target). However, it appears that if <code>VAR</code> variable isn't defined in parent, child gets an empty string (it has <code>VAR ?= ...</code>, and the value of <code>VAR</code> is empty). How can I avoid this? </p>
  
  <p>It looks like a variation on <a href="https://stackoverflow.com/questions/1909188/define-make-variable-at-rule-execution-time">Define make variable at rule execution time</a> should work for my specific usecase, but it requires the parent Makefile to know child's default value, which I want to avoid.</p>