title: How to append values in xcconfig variables?
body: <p>I'm using Xcode and .xcconfig files.  I'm trying to append some values in the preprocessor definitions, but I simply can't make it work.</p>
  
  <p>I tried the following (as well as many variations of this), but no luck so far:</p>
  
  <p><code>GCC_PREPROCESSOR_DEFINITIONS = '$(GCC_PREPROCESSOR_DEFINITIONS) NEW_VALUE'</code></p>
  
  <p>The <code>NEW_VALUE</code> symbol is simply never added to the preprocessor definitions.</p>
  
  <p>Does anyone had success appending new values to variables in xcconfig files?</p>