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_tbox.cpp

@@ -10,6 +10,7 @@
 #include "SSystem/SComponent/c_math.h"
 #include "Z2AudioLib/Z2Instances.h"
 #include <cmath>
+#include <cstring>
 
 static const f32 l_cull_size_box[6] = { -150.0f, -10.0f, -150.0f, 150.0f, 300.0f, 100.0f };