Enable C++17

All supported platforms now have easy access to a compiler with C++17
support.

C++17 potentially allows for some nice cleanups and removes the need
for standard library backports (optional/variant).

See discussion at https://dolp.in/pr6264#discussion_r158134178

Source/CMakeLists.txt

@@ -16,18 +16,13 @@ if(CMAKE_C_COMPILER_ID MATCHES "MSVC")
   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.
+  # Enable C++17
+  # CMAKE_CXX_STANDARD cannot be used because it requires cmake 3.8+.
   check_and_add_flag(CXX17 -std=c++17)
   if(NOT FLAG_CXX_CXX17)
-    set(CMAKE_CXX_STANDARD 14)
-    set(CMAKE_CXX_STANDARD_REQUIRED ON)
-    set(CMAKE_CXX_EXTENSIONS OFF)
+    # Fall back to -std=c++1z
+    check_and_add_flag(CXX1Z -std=c++1z)
   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

Source/Core/Common/Compat/in_place.h

@@ -1,45 +0,0 @@
-// 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 mpark
-{
-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;
-};
-
-#ifdef MPARK_VARIABLE_TEMPLATES
-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{};
-#endif
-
-}  // namespace mpark