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/d/actor/d_a_npc_ne.cpp

@@ -18,6 +18,7 @@
 #include "f_op/f_op_kankyo_mng.h"
 #include "c/c_damagereaction.h"
 #include "Z2AudioLib/Z2Instances.h"
+#include <cstring>
 
 static home_path_pnt home_path[38] = {
     {0, {561.0f, 87.0f, -1110.0f}},