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_debug_camera.cpp

@@ -8,6 +8,7 @@
 #include "JSystem/JHostIO/JORServer.h"
 #include "JSystem/JUtility/JUTDbPrint.h"
 #include "JSystem/JUtility/JUTReport.h"
+#include <cstring>
 
 namespace zevwork {
     const int CutMaxNum = 32;