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/Z2SoundObjMgr.cpp

@@ -4,6 +4,7 @@
 #include "Z2AudioLib/Z2Calc.h"
 #include "Z2AudioLib/Z2SceneMgr.h"
 #include "Z2AudioLib/Z2LinkMgr.h"
+#include <cstring>
 
 Z2SoundObjMgr::Z2SoundObjMgr() : JASGlobalInstance<Z2SoundObjMgr>(true) {
     ghostEnemyState_ = 0;