title: Define a preprocessor value from command line using MSBuild
body: <p>I need to create a demo version of an existing large application consisting of multiple projects.  I'd like to use the existing projects, and just neuter the functionality via preprocessor directives </p>
  
  <pre><code>#if DEMO
      mycode.NeuterNow();
  #endif
  </code></pre>
  
  <p>We are building our app using MSBuild, and I'd ideally use something along the lines of:</p>
  
  <pre><code>MSBuild -DefineProperty:DEMO MySolution.sln
  </code></pre>
  
  <p>Does such functionality exist?</p>