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_gra.cpp

@@ -12,6 +12,7 @@
 #include "d/actor/d_a_npc_gra.h"
 #include "d/actor/d_a_tag_gra.h"
 #include "Z2AudioLib/Z2Instances.h"
+#include <cstring>
 
 const daNpc_grA_HIOParam daNpc_grA_Param_c::m = {
     {90.0f,  -4.0f,  1.0f,   850.0f,  255.0f, 280.0f, 40.0f, 100.0f, 0.0f, 0.0f, 20.0f,