Some GCC compilation fixes (#3114)

* Fix 6-byte multichar literals

* Add `struct` to `e_ga_class::ga_s`

* Fix remaining wrong forward declares (struct/class)

* Replace `#include <string>` with `#include <cstring>`

* Guard FLT_EPSILON define to prevent redefinition

* Add missing `#include <cstring>` for direct cstring function usage

src/Z2AudioLib/Z2SceneMgr.cpp

@@ -11,6 +11,7 @@
 #include "Z2AudioLib/Z2SoundObjMgr.h"
 #include "Z2AudioLib/Z2StatusMgr.h"
 #include "d/d_com_inf_game.h"
+#include <cstring>
 
 Z2SceneMgr::Z2SceneMgr() : JASGlobalInstance<Z2SceneMgr>(true) {
     sceneNum = -1;