title: Test if a C macro's value is empty
body: <p>I need to write some code to verify that a macro is defined but empty (not having any values). The test does not need to be in compile time.</p>
  
  <p>I am attempting to write:</p>
  
  <pre><code>#if (funcprototype == "")
  MY_WARN("funcprototype is empty");
  #endif
  </code></pre>
  
  <p>the code does not compile, as <code>funcprototype</code> expands to empty.</p>