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_kbacket.cpp

@@ -11,6 +11,7 @@
 #include "d/actor/d_a_player.h"
 #include "d/d_cc_d.h"
 #include "d/d_com_inf_game.h"
+#include <cstring>
 
 static s32 l_bmdData[4] = { 8, 0, 9, 0 };