Fix build on platforms without std::string_view

json.hpp wants it, but it isn't always available. Rather than patch
json.hpp directly to remove the dependency, provide a json.h wrapper
header that subs in std::experimental::string_view using preprocessor
magic. All the consumers of json.hpp are in src/web_service, so the
wrapper header is placed there as well.

src/web_service/announce_room_json.cpp

@@ -3,9 +3,9 @@
 // Refer to the license.txt file included.
 
 #include <future>
-#include <json.hpp>
 #include "common/logging/log.h"
 #include "web_service/announce_room_json.h"
+#include "web_service/json.h"
 #include "web_service/web_backend.h"
 
 namespace AnnounceMultiplayerRoom {

src/web_service/json.h

@@ -0,0 +1,18 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+// This hack is needed to support json.hpp on platforms where the C++17 stdlib
+// lacks std::string_view. See https://github.com/nlohmann/json/issues/735.
+// clang-format off
+#if !__has_include(<string_view>) && __has_include(<experimental/string_view>)
+# include <experimental/string_view>
+# define string_view experimental::string_view
+# include <json.hpp>
+# undef string_view
+#else
+# include <json.hpp>
+#endif
+// clang-format on

src/web_service/telemetry_json.h

@@ -7,9 +7,9 @@
 #include <array>
 #include <future>
 #include <string>
-#include <json.hpp>
 #include "common/announce_multiplayer_room.h"
 #include "common/telemetry.h"
+#include "web_service/json.h"
 
 namespace WebService {
 

src/web_service/verify_login.cpp

@@ -2,7 +2,7 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
-#include <json.hpp>
+#include "web_service/json.h"
 #include "web_service/verify_login.h"
 #include "web_service/web_backend.h"