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_e_bug.cpp

@@ -10,6 +10,7 @@
 #include "d/d_path.h"
 #include "d/actor/d_a_nbomb.h"
 #include "Z2AudioLib/Z2Instances.h"
+#include <cstring>
 
 enum E_bug_RES_File_ID {
     /* BMDG */