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/d_menu_fmap2D.cpp

@@ -17,6 +17,7 @@
 #include "d/d_msg_scrn_explain.h"
 #include "m_Do/m_Do_graphic.h"
 #include "d/actor/d_a_midna.h"
+#include <cstring>
 
 dMenu_Fmap2DBack_c::dMenu_Fmap2DBack_c() {
     dMeter2Info_setMapDrugFlag(0);