build/install: fall back to Unix desktop

Instead of enumerating every Unix assume Freedesktop-compliant behavior.
In future Android may want to define its own flavor like OSX.

OpenRA.Game/Platform.cs

@@ -36,10 +36,10 @@ namespace OpenRA
 				psi.RedirectStandardOutput = true;
 				var p = Process.Start(psi);
 				var kernelName = p.StandardOutput.ReadToEnd();
-				if (kernelName.Contains("Linux") || kernelName.Contains("BSD"))
-					return PlatformType.Linux;
 				if (kernelName.Contains("Darwin"))
 					return PlatformType.OSX;
+				else
+					return PlatformType.Linux;
 			}
 			catch { }