title: Custom command line parameter in Inno Setup with default value so I can pass build configuration
body: <p>I'm working with an Inno Setup .iss file to create an installer for a .NET project.<br />
  I want to default the installer build to use the Debug build output paths so devs can create local installer for testing, but I want to be able to pass a parameter to the Inno compiler to tell it to use the Release output paths instead when doing a proper production build.</p>
  <p>From what I can gather I can't add a custom variable to the <code>[Setup]</code> section, because I tried</p>
  <pre><code>[Setup]  
  Config = {param:config|Debug}
  </code></pre>
  <p>as described in <a href="https://stackoverflow.com/q/3618257/">Is it possible to accept custom command line parameters with Inno Setup</a>, but I get</p>
  <blockquote>
  <p>Unrecognized [Setup] section directive &quot;Config&quot;</p>
  </blockquote>
  <p>So it appears to me that the only option is to have something in the <code>#define</code> section.</p>
  <p>I see that it's possible to define parameters at the command line with <code>/Dparamname=value</code>.
  So I tried the <code>{param:Config|Debug}</code> and <code>{param:#Config|Debug}</code> as suggested in <a href="https://stackoverflow.com/q/2071826/">Inno Setup - Conditional Parameters</a>, but again I get compiler errors:</p>
  <blockquote>
  <p>Unknown filename prefix &quot;..\project\bin\{param:&quot;</p>
  </blockquote>
  <p>And if I <code>#define Config &quot;Debug&quot;</code> in the iss file then passing <code>/DConfig=Release</code> at the command line doesn't override the Debug value.</p>
  <p>So is there any neat way to do what I want, where I can run the compile with a default value if nothing is passed into the command line, and use the command line value if it's available?<br />
  Or is there an altogether easier way to do this that I'm missing.</p>
  <p>Currently my command line call looks like this:</p>
  <pre><code>iscc path\to\my\iss\file /DConfig=Release
  </code></pre>
  <p>I'm using Inno Setup 5.5.9.</p>