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/J2DGraph/J2DPrint.cpp

@@ -5,6 +5,7 @@
 #include "JSystem/JUtility/JUTFont.h"
 #include <cstdio>
 #include <cstdlib>
+#include <cstring>
 
 char* J2DPrint::mStrBuff;
 

src/JSystem/J2DGraph/J2DTextBox.cpp

@@ -5,6 +5,7 @@
 #include "JSystem/JSupport/JSURandomInputStream.h"
 #include "JSystem/JUtility/JUTResFont.h"
 #include "JSystem/JUtility/JUTResource.h"
+#include <cstring>
 
 J2DTextBox::J2DTextBox()
     : mFont(NULL), mCharColor(), mGradientColor(), mStringPtr(NULL), mWhiteColor(), mBlackColor() {

src/JSystem/JAHostIO/JAHioNode.cpp

@@ -4,6 +4,7 @@
 #include "JSystem/JAHostIO/JAHioMessage.h"
 #include "JSystem/JAHostIO/JAHioMgr.h"
 #include "JSystem/JHostIO/JORServer.h"
+#include <cstring>
 
 JAHioNode::JAHioNode(const char* name) : mTree(this) {
     mLastChild = NULL;

src/JSystem/JAudio2/JAUSectionHeap.cpp

@@ -15,6 +15,7 @@
 #include "JSystem/JAudio2/JAUStreamFileTable.h"
 #include "JSystem/JKernel/JKRSolidHeap.h"
 #include <dolphin/dvd.h>
+#include <cstring>
 
 namespace {
     class TPushCurrentHeap {

src/JSystem/JUtility/JUTCacheFont.cpp

@@ -7,6 +7,7 @@
 #include "JSystem/JKernel/JKRAram.h"
 #include <dolphin/gx.h>
 #include <stdint.h>
+#include <cstring>
 #include "angle_utils.h"
 
 JUTCacheFont::JUTCacheFont(ResFONT const* p_fontRes, u32 cacheSize, JKRHeap* p_heap) {

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;

src/JSystem/JUtility/JUTException.cpp

@@ -9,6 +9,7 @@
 #include <cstdlib>
 #include <dolphin/dolphin.h>
 #include <stdint.h>
+#include <cstring>
 
 struct CallbackObject {
     /* 0x00 */ JUTExceptionUserCallback callback;