title: Mysterious Qt undefined interface : investigation
body: <p>I am writing a typical Qt (4.8.2) app with plugins. One of the plugins includes a couple custom interfaces, by calling the Q_INTERFACE macro. Those interfaces are found in .h files visible to the project (via the .pro includes) and they all contain a macro call to Q_DECLARE_INTERFACES.</p>
  
  <p>Yet upon compiling the <strong>moc file</strong> I get an <strong>undefined interface error</strong> in the main .h file of the plugin. It doesn't tell me which though, it only specifies the line of the first (I tried changing the order but it makes no difference - unless <em>all</em> interfaces are affected - heck, who knows, that could be the case.)</p>
  
  <p>This plugin works at other people's desks. Some of the stuff in the main app is written with Qt 5 but always with macros to check for the version and include other code for earlier versions like mine. This also works at other people's desks who work with 4.8.2 for their plugins.</p>
  
  <p>Where to look for the cause of that error ?</p>