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/actor/d_a_bg.cpp

@@ -16,6 +16,7 @@
 #include "JSystem/JKernel/JKRExpHeap.h"
 #include "JSystem/JKernel/JKRSolidHeap.h"
 #include "JSystem/J3DGraphAnimator/J3DMaterialAnm.h"
+#include <cstring>
 
 const char* daBg_c::setArcName() {
     static char arcName[32];