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/d_kankyo_debug.cpp

@@ -7,6 +7,7 @@
 #include "f_op/f_op_camera_mng.h"
 #include "m_Do/m_Do_controller_pad.h"
 #include "m_Do/m_Do_graphic.h"
+#include <cstring>
 
 static void add_update_proc(u8* mode, s16* param_1, s16* param_2, s16* param_3) {
     int value;