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.

cmake/Options.cmake

@@ -71,6 +71,8 @@ function(set_options)
     if (OPTION_DOCS_ONLY)
         set(OPTION_TOOLS_ONLY ON PARENT_SCOPE)
     endif()
+
+    option(OPTION_SURVEY_KEY "Survey-key to use for the opt-in survey (empty if you have none)" "")
 endfunction()
 
 # Show the values of the generic options.
@@ -84,6 +86,12 @@ function(show_options)
     message(STATUS "Option Use assert - ${OPTION_USE_ASSERTS}")
     message(STATUS "Option Use threads - ${OPTION_USE_THREADS}")
     message(STATUS "Option Use NSIS - ${OPTION_USE_NSIS}")
+
+    if(OPTION_SURVEY_KEY)
+        message(STATUS "Option Survey Key - USED")
+    else()
+        message(STATUS "Option Survey Key - NOT USED")
+    endif()
 endfunction()
 
 # Add the definitions for the options that are selected.
@@ -104,4 +112,8 @@ function(add_definitions_based_on_options)
     else()
         add_definitions(-DNDEBUG)
     endif()
+
+    if(OPTION_SURVEY_KEY)
+        add_definitions(-DSURVEY_KEY="${OPTION_SURVEY_KEY}")
+    endif()
 endfunction()