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

include/d/d_msg_object.h

@@ -4,6 +4,7 @@
 #include "JSystem/JMessage/control.h"
 #include "d/d_com_inf_game.h"
 #include "f_op/f_op_msg.h"
+#include <cstring>
 
 class COutFont_c;
 class J2DTextBox;