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_msg_string_base.cpp

@@ -10,6 +10,7 @@
 #include "d/d_meter2_info.h"
 #include "JSystem/JKernel/JKRArchive.h"
 #include "JSystem/J2DGraph/J2DTextBox.h"
+#include <cstring>
 
 dMsgStringBase_c::dMsgStringBase_c() {
     field_0x20 = 0;