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_so.cpp

@@ -15,6 +15,7 @@
 #include "d/d_cc_uty.h"
 #include "d/d_s_play.h"
 #include "f_op/f_op_camera_mng.h"
+#include <cstring>
 
 daObj_So_HIO_c::daObj_So_HIO_c() {
     field_0x4 = -1;