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/JSystem/JUtility/JUTDbPrint.cpp

@@ -5,6 +5,7 @@
 #include "JSystem/JKernel/JKRHeap.h"
 #include "JSystem/JUtility/JUTVideo.h"
 #include <cstdio>
+#include <cstring>
 
 JUTDbPrint::JUTDbPrint(JUTFont* pFont, JKRHeap* pHeap) {
     mFont = pFont;