FIxed GDC compilation (basic betterC WIP) and some improvements

-fixed issue with adding/removing entities inside events handling
-fixed EntityMeta.getComponent() (added check if component_id is valid)
-added function hasComponent to entity to check if component exists

demos/external/sources/mmutils/thread_pool.d

@@ -182,6 +182,12 @@ void instructionPause()
 
 			__builtin_ia32_pause();
 		}
+		else version(GNU)
+		{
+			import gcc.builtins;
+
+			__builtin_ia32_pause();
+		}
 		else version (DigitalMars)
 		{
 			asm
@@ -189,7 +195,6 @@ void instructionPause()
 				rep;
 				nop;
 			}
-
 		}
 		else
 		{