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_obj_gb.cpp

@@ -10,6 +10,7 @@
 #include "SSystem/SComponent/c_math.h"
 #include "d/d_bg_w.h"
 #include "d/d_com_inf_game.h"
+#include <cstring>
 
 static int daObj_Gb_Draw(obj_gb_class* i_this) {
     g_env_light.settingTevStruct(0x10, &i_this->current.pos, &i_this->tevStr);