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_ystone.cpp

@@ -8,6 +8,7 @@
 #include "d/actor/d_a_obj_ystone.h"
 #include "d/d_com_inf_game.h"
 #include "d/d_procname.h"
+#include <cstring>
 
 static char const* l_arcName[7] = {
     "N_gD_mskF",