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_meter_string.cpp

@@ -16,6 +16,7 @@
 #include "d/d_meter2_info.h"
 #include "d/d_meter_HIO.h"
 #include "d/d_pane_class.h"
+#include <cstring>
 
 dMeterString_c::dMeterString_c(int i_stringID) {
     mpMapArchive = dComIfGp_getAllMapArchive();