Feature: opt-in survey when exiting a game

On first start-up, the game will ask if you want to participate
in our automated survey. You have to opt-in, and can easily opt-out
(via the Options) at any time.

When opt-in, whenever you exit a game, a JSON blob will be send
to the survey server hosted by OpenTTD. This JSON blob contains
information that gives a global picture of the game just played:
- What settings were used
- How many humans vs AIs
- How long the game has been played
- Basic information about the OS / CPU

All this information is kept very generic, so there is no
chance we send private information to our survey server.
Nothing in the JSON blob could identify you as a person; it
mostly tells about the game played. At any time you can see
what the JSON blob includes, by pressing the "Preview Survey
Results" button in-game.

src/os/macosx/survey_osx.cpp

@@ -0,0 +1,38 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file survey_osx.cpp OSX implementation of OS-specific survey information. */
+
+#ifdef WITH_NLOHMANN_JSON
+
+#include "../../stdafx.h"
+
+#include "../../3rdparty/fmt/format.h"
+#include "macos.h"
+
+#include <mach-o/arch.h>
+#include <nlohmann/json.hpp>
+
+#include "../../safeguards.h"
+
+void SurveyOS(nlohmann::json &json)
+{
+	int ver_maj, ver_min, ver_bug;
+	GetMacOSVersion(&ver_maj, &ver_min, &ver_bug);
+
+	const NXArchInfo *arch = NXGetLocalArchInfo();
+
+	json["os"] = "MacOS";
+	json["release"] = fmt::format("{}.{}.{}", ver_maj, ver_min, ver_bug);
+	json["machine"] = arch != nullptr ? arch->description : "unknown";
+	json["min_ver"] = MAC_OS_X_VERSION_MIN_REQUIRED;
+	json["max_ver"] = MAC_OS_X_VERSION_MAX_ALLOWED;
+
+	json["memory"] = MacOSGetPhysicalMemory();
+}
+
+#endif /* WITH_NLOHMANN_JSON */