Use defined for some preprocessor variables that might be undefined

The variables PYBIND11_HAS_OPTIONAL, PYBIND11_HAS_EXP_OPTIONAL, PYBIND11_HAS_VARIANT,
__clang__, __APPLE__ were not checked for defined in a minortity of instances.

If the project using pybind11 sets -Wundef, the warnings will show.

The test build is also modified to catch the problem.

The errors looked like:

```
ext/pybind11/include/pybind11/stl.h:292:5: error: "PYBIND11_HAS_OPTIONAL" is not defined, evaluates to 0 [-Werror=undef]
  292 | #if PYBIND11_HAS_OPTIONAL
      |     ^~~~~~~~~~~~~~~~~~~~~
ext/pybind11/include/pybind11/stl.h:300:5: error: "PYBIND11_HAS_EXP_OPTIONAL" is not defined, evaluates to 0 [-Werror=undef]
  300 | #if PYBIND11_HAS_EXP_OPTIONAL
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
ext/pybind11/include/pybind11/stl.h:372:5: error: "PYBIND11_HAS_VARIANT" is not defined, evaluates to 0 [-Werror=undef]
  372 | #if PYBIND11_HAS_VARIANT
      |     ^~~~~~~~~~~~~~~~~~~~
```

Author: cirosantilli2

include/pybind11/stl.h

@@ -289,15 +289,15 @@ template<typename T> struct optional_caster {
     PYBIND11_TYPE_CASTER(T, _("Optional[") + value_conv::name + _("]"));
 };
 
-#if PYBIND11_HAS_OPTIONAL
+#if defined(PYBIND11_HAS_OPTIONAL)
 template<typename T> struct type_caster<std::optional<T>>
     : public optional_caster<std::optional<T>> {};
 
 template<> struct type_caster<std::nullopt_t>
     : public void_caster<std::nullopt_t> {};
 #endif
 
-#if PYBIND11_HAS_EXP_OPTIONAL
+#if defined(PYBIND11_HAS_EXP_OPTIONAL)
 template<typename T> struct type_caster<std::experimental::optional<T>>
     : public optional_caster<std::experimental::optional<T>> {};
 
@@ -369,7 +369,7 @@ struct variant_caster<V<Ts...>> {
     PYBIND11_TYPE_CASTER(Type, _("Union[") + detail::concat(make_caster<Ts>::name...) + _("]"));
 };
 
-#if PYBIND11_HAS_VARIANT
+#if defined(PYBIND11_HAS_VARIANT)
 template <typename... Ts>
 struct type_caster<std::variant<Ts...>> : variant_caster<std::variant<Ts...>> { };
 #endif

tests/CMakeLists.txt

@@ -197,7 +197,7 @@ function(pybind11_enable_warnings target_name)
     target_compile_options(${target_name} PRIVATE /W4)
   elseif(CMAKE_CXX_COMPILER_ID MATCHES "(GNU|Intel|Clang)")
     target_compile_options(${target_name} PRIVATE -Wall -Wextra -Wconversion -Wcast-qual
-                                                  -Wdeprecated)
+                                                  -Wdeprecated -Wundef)
   endif()
 
   if(PYBIND11_WERROR)

tests/test_operator_overloading.cpp

@@ -88,11 +88,11 @@ std::string abs(const Vector2&) {
   // Here, we suppress the warning using `#pragma diagnostic`.
   // Taken from: https://github.com/RobotLocomotion/drake/commit/aaf84b46
   // TODO(eric): This could be resolved using a function / functor (e.g. `py::self()`).
-  #if (__APPLE__) && (__clang__)
+  #if defined(__APPLE__) && defined(__clang__)
     #if (__clang_major__ >= 10) && (__clang_minor__ >= 0) && (__clang_patchlevel__ >= 1)
       #pragma GCC diagnostic ignored "-Wself-assign-overloaded"
     #endif
-  #elif (__clang__)
+  #elif defined(__clang__)
     #if (__clang_major__ >= 7)
       #pragma GCC diagnostic ignored "-Wself-assign-overloaded"
     #endif