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_msg_flow.cpp

@@ -13,6 +13,7 @@
 #include "d/actor/d_a_myna.h"
 #include "d/actor/d_a_obj_ss_base.h"
 #include "SSystem/SComponent/c_math.h"
+#include <cstring>
 
 dMsgFlow_c::dMsgFlow_c() {
     mNonStopJunpFlowFlag = 0;