title: How to optionally include certain code for certain features?
body: <p>In Inno Setup, I have a main script which is the "core system", meaning everything which is absolutely needed for our software to install/run at all. Additionally, I'm writing script files for each major feature which may or may not be compiled into the installer. At the top of the main script file, I include other script files...</p>
  
  <pre><code>#include "AdditionalOption.iss"
  #include "AnotherOption.iss"
  </code></pre>
  
  <p>When compiling this main script, the person compiling may choose whether or not to compile these certain options in the installer at all (to spare file size for various reasons).</p>
  
  <p>The problem arises when I have specific code in the main script which depends on something in one of these additional scripts. For example...</p>
  
  <pre><code>procedure InitializeWizard();
  begin
    //Creates custom wizard page only if "AdditionalOption.iss" is compiled
    CreatePageForAdditionalOption; 
    //Creates custom wizard page only if "AnotherOption.iss" is compiled
    CreatePageForAnotherOption; 
  end;
  </code></pre>
  
  <p><code>InitializeWizard</code> can only be defined once, but I need to call code in it conditionally depending on whether or not those other scripts were included. Those procedures reside in their appropriate script files, so of course they don't exist if user excluded that other script file.</p>
  
  <p>In Delphi, I can use conditionals like so:</p>
  
  <pre><code>{$DEFINE ADD_OPT}
  {$DEFINE ANO_OPT}
  
  procedure InitializeWizard();
  begin
    {$IFDEF ADD_OPT}
    CreatePageForAdditionalOption;
    {$ENDIF}
    {$IFDEF ANO_OPT}
    CreatePageForAnotherOption;
    {$ENDIF}
  end;
  </code></pre>
  
  <p>Of course this isn't actually Delphi though. How can I do such a thing in Inno Setup?</p>