SGSubsystem classes: Registration of all subsystems added since mid 2018.

src/GUI/Highlight.cxx

@@ -668,3 +668,7 @@ void Highlight::addPropertyProperty(const std::string& from0, const std::string&
         }
     }
 }
+
+// Register the subsystem.
+SGSubsystemMgr::Registrant<Highlight> registrantHighlight(
+    SGSubsystemMgr::INIT);

src/Main/ErrorReporter.cxx

@@ -942,3 +942,7 @@ std::string ErrorReporter::threadSpecificContextValue(const std::string& key)
 
 
 } // namespace flightgear
+
+// Register the subsystem.
+SGSubsystemMgr::Registrant<flightgear::ErrorReporter> registrantErrorReporter(
+    SGSubsystemMgr::GENERAL);

src/Viewer/GraphicsPresets.cxx

@@ -547,4 +547,9 @@ bool GraphicsPresets::saveToXML(const SGPath& path, const std::string& name, con
     return true;
 }
 
+// Register the subsystem.
+SGSubsystemMgr::Registrant<GraphicsPresets> registrantGraphicsPresets(
+    SGSubsystemMgr::DISPLAY);
+
+
 } // namespace flightgear