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

include/JSystem/JGeometry.h

@@ -7,8 +7,10 @@
 
 #ifndef __MWERKS__
 #include <limits>
+#if !defined(FLT_EPSILON)
 #define FLT_EPSILON std::numeric_limits<float>::epsilon()
 #endif
+#endif
 
 namespace JGeometry {