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_picture.cpp

@@ -6,6 +6,7 @@
 #include "d/dolzel_rel.h" // IWYU pragma: keep
 
 #include "d/actor/d_a_obj_picture.h"
+#include <cstring>
 
 static int daObjPicture_c_createHeap(fopAc_ac_c* i_this) {
     return ((daObjPicture_c*)i_this)->createHeap();