Gui: Selectively disable MSVC 4251 warning

This warning is not relevant to our project in this instance. We are not concerned about ABI-compatibility of DLLs since they are all compiled at the same time when building FreeCAD.

src/Gui/DocumentObserver.h

@@ -306,6 +306,11 @@ private:
     ViewProviderWeakPtrT ptr;
 };
 
+#ifdef _MSC_VER
+#  pragma warning(push)
+#  pragma warning(disable: 4251)  // MSVC emits warning C4251 too conservatively for our use-case
+#endif
+
 /**
  * The DocumentObserver class simplifies the step to write classes that listen
  * to what happens inside a document.
@@ -368,6 +373,10 @@ private:
     Connection connectDocumentDelete;
 };
 
+#ifdef _MSC_VER
+#  pragma warning(pop)
+#endif
+
 } //namespace Gui
 
 #endif // GUI_DOCUMENTOBSERVER_H