Avoid LNK2001 in c++20 when /GL (Whole program optimization) is ON with MSVC

[hellokenlee]

The following c++20 code would not link when using /GL in MSVC:

class __declspec(dllexport) Animal {
public:
	virtual ~Animal() = default;
	virtual void go(int n_times) {}
};

class __declspec(dllexport) PyAnimal : public Animal, public pybind11::trampoline_self_life_support {
public:
	void go(int n_times) override {
		PYBIND11_OVERRIDE_PURE(
			void,
			Animal,
			go,
			n_times
		);
	}
};

PYBIND11_MODULE(example, m, pybind11::mod_gil_not_used()) {
	pybind11::class_<Animal, PyAnimal, pybind11::smart_holder>(m, "Animal")
		.def(pybind11::init<>())
		.def("go", &Animal::go)
	;
}

The error is:

Error LNK2001 : unresolved external symbol "struct pybind11::detail::override_unused `public: virtual void __cdecl PyAnimal::go(int)'::`12'::caster" (?caster@?M@??go@PyAnimal@@UEAAXH@Z@4Uoverride_unused@detail@pybind11@@A)

We can use constexpr if to avoid this link error.