Make FGViewer::update() a pure virtual because FGViewer is a base class

and can never be instantiated itself.

src/Main/viewer.hxx

@@ -116,7 +116,7 @@ protected:
     inline void set_clean() { dirty = false; }
 
     // Update the view volume, position, and orientation
-    virtual void update();
+    virtual void update() = 0;
 
 public:
 

src/Scenery/newcache.cxx

@@ -67,9 +67,9 @@ void FGNewCache::entry_free( long cache_index ) {
     FGTileEntry *tile = tile_cache[cache_index];
     tile->disconnect_ssg_nodes();
 
-#ifdef WISH_PLIB_WAS_THREADED
+#ifdef WISH_PLIB_WAS_THREADED // but it isn't
     tile->sched_removal();
-#else // plib isn't threaded so we always go here
+#else
     tile->free_tile();
     delete tile;
 #endif