Common: Add a std::optional implementation

std::optional makes a few things a bit neater and less error prone.
However, we still cannot use C++17 (unfortunately), so this commit
adds an implementation of std::optional that we can use right now.

Based on https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/lib/gtl/optional.h
which seems to be fairly similar to C++17's <optional> and standards
compliant. It's one of the few implementations that handle propagating
type traits like copy constructibility, just like libc++/libstdc++.

Source/CMakeLists.txt

@@ -1,7 +1,3 @@
-set(CMAKE_CXX_STANDARD 14)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
-
 if(CMAKE_SYSTEM_NAME MATCHES "Windows")
   add_definitions(-DNOMINMAX)
   add_definitions(-DUNICODE)
@@ -13,11 +9,29 @@ if(CMAKE_SYSTEM_NAME MATCHES "Windows")
   add_definitions(-D_CRT_SECURE_NO_DEPRECATE)
 endif()
 
-# enable the latest C++ standard feature set,
-# and also disable MSVC specific extensions
-# to be even more standards compliant.
-check_and_add_flag(CPPLATEST /std:c++latest)
-check_and_add_flag(STANDARD_COMPLIANCE /permissive-)
+if(CMAKE_C_COMPILER_ID MATCHES "MSVC")
+  # enable the latest C++ standard feature set,
+  # and also disable MSVC specific extensions
+  # to be even more standards compliant.
+  check_and_add_flag(CPPLATEST /std:c++latest)
+  check_and_add_flag(STANDARD_COMPLIANCE /permissive-)
+else()
+  # Enable C++17, but fall back to C++14 if it isn't available.
+  # CMAKE_CXX_STANDARD cannot be used here because we require C++14 or newer, not any standard.
+  check_and_add_flag(CXX17 -std=c++17)
+  if(NOT FLAG_CXX_CXX17)
+    check_and_add_flag(CXX1Z -std=c++1z)
+    if (NOT FLAG_CXX_CXX1Z)
+      set(CMAKE_CXX_STANDARD 14)
+      set(CMAKE_CXX_STANDARD_REQUIRED ON)
+      set(CMAKE_CXX_EXTENSIONS OFF)
+    endif()
+  endif()
+
+  # These compat headers must not be in the include path when building with MSVC,
+  # because it currently does not support __has_include_next / #include_next.
+  include_directories(SYSTEM Core/Common/Compat)
+endif()
 
 # These aren't actually needed for C11/C++11
 # but some dependencies require them (LLVM, libav).

Source/Core/Common/Common.vcxproj

@@ -227,4 +227,4 @@
   <Import Project="$(VCTargetsPath)\Microsoft.Cpp.targets" />
   <ImportGroup Label="ExtensionTargets">
   </ImportGroup>
-</Project>
+</Project>

Source/Core/Common/Common.vcxproj.filters

@@ -324,4 +324,4 @@
   <ItemGroup>
     <Natvis Include="BitField.natvis" />
   </ItemGroup>
-</Project>
+</Project>

Source/Core/Common/Compat/in_place.h

@@ -0,0 +1,43 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+// MPark.Variant
+//
+// Copyright Michael Park, 2015-2017
+//
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
+
+#include <cstddef>
+
+namespace std
+{
+struct in_place_t
+{
+  explicit in_place_t() = default;
+};
+
+template <std::size_t I>
+struct in_place_index_t
+{
+  explicit in_place_index_t() = default;
+};
+
+template <typename T>
+struct in_place_type_t
+{
+  explicit in_place_type_t() = default;
+};
+
+constexpr in_place_t in_place{};
+
+template <std::size_t I>
+constexpr in_place_index_t<I> in_place_index{};
+
+template <typename T>
+constexpr in_place_type_t<T> in_place_type{};
+
+}  // namespace std