Add version command line option. Fixes #2965.

This was SVN commit r17611.

binaries/system/readme.txt

@@ -39,6 +39,7 @@ Configuration:
 -yres=N				set screen Y resolution to 'N'
 
 Advanced / diagnostic:
+-version			print the version of the engine and exit
 -dumpSchema			creates a file entity.rng in the working directory, containing
 					  complete entity XML schema, used by various analysis tools
 -entgraph			(disabled)

source/main.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2015 Wildfire Games.
+/* Copyright (C) 2016 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -416,6 +416,12 @@ static void RunGameOrAtlas(int argc, const char* argv[])
 
 	g_args = args;
 
+	if (args.Has("version") || args.Has("-version"))
+	{
+		debug_printf("Pyrogenesis %s\n", engine_version);
+		return;
+	}
+
 	// We need to initialise libxml2 in the main thread before
 	// any thread uses it. So initialise it here before we
 	// might run Atlas.