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_calibration.cpp

@@ -12,6 +12,7 @@
 #include "d/d_meter_HIO.h"
 #include "d/d_msg_string.h"
 #include "m_Do/m_Do_controller_pad.h"
+#include <cstring>
 
 // Need 0xC bytes of padding with no symbol between dMenu_Calibration_c::__vtable and the end of .data
 // This is likely caused by the vtable of an abstract base class getting put there and then stripped out.