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_event_data.cpp

@@ -13,6 +13,7 @@
 #include "d/d_msg_object.h"
 #include "m_Do/m_Do_graphic.h"
 #include "SSystem/SComponent/c_counter.h"
+#include <cstring>
 
 inline BOOL dEvDtFlagCheck(int i_flag) {
     return dComIfGp_getEventManager().getFlags().flagCheck(i_flag);