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_ihasi.cpp

@@ -7,6 +7,7 @@
 
 #include "d/actor/d_a_obj_ihasi.h"
 #include "d/d_com_inf_game.h"
+#include <cstring>
 
 static int daObj_Ihasi_Draw(obj_ihasi_class* i_this) {
     fopAc_ac_c* a_this = (fopAc_ac_c*)i_this;