Identifiers containing double underscore are reserved according to the C++ standard

Rename __* to _gde_*

https://timsong-cpp.github.io/cppwp/n3337/global.names

https://en.cppreference.com/w/cpp/language/identifiers

Identifiers appearing as a token or preprocessing token (i.e., not in user-defined-string-literal like operator ""id) (since C++11) of one of the following forms are reserved:
 - identifiers with a double underscore anywhere;
 - identifiers that begin with an underscore followed by an uppercase letter;
 - in the global namespace, identifiers that begin with an underscore.

include/godot_cpp/core/method_bind.hpp

@@ -268,8 +268,8 @@ MethodBind *create_vararg_method_bind(R (T::*p_method)(const Variant **, GDExten
 }
 
 #ifndef TYPED_METHOD_BIND
-class ___UnexistingClass;
-#define MB_T ___UnexistingClass
+class _gde_UnexistingClass;
+#define MB_T _gde_UnexistingClass
 #else
 #define MB_T T
 #endif