Various debug conditional compilation cleanup (#2915)

* Global: Define DEBUG as 0 if not already defined

* Clean up DEBUG-guarded code

include/d/actor/d_a_obj_fallobj.h

@@ -68,7 +68,7 @@ private:
 
 STATIC_ASSERT(sizeof(daObjFallObj_c) == 0x614);
 
-#ifdef DEBUG
+#if DEBUG
 class daObjFallObj_HIO_c : public mDoHIO_entry_c {
 public:
     daObjFallObj_HIO_c();