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_alink.cpp

@@ -52,6 +52,7 @@
 #include "d/d_s_play.h"
 
 #include "res/Object/Alink.h"
+#include <cstring>
 
 static int daAlink_Create(fopAc_ac_c* i_this);
 static int daAlink_Delete(daAlink_c* i_this);