cli11: prefer system package if found

mu embeds a "vendored" version of libcl11 under thirdparty/; but it's
better to use the system-package if the user has one.

So, use the system package if found or user -Duse-embedded-cli11=true is
specified.

mu/mu-options.cc

@@ -49,7 +49,11 @@
 #include "mu-options.hh"
 #include "mu-script.hh"
 
-#include <thirdparty/CLI11.hpp>
+#ifdef USE_EMBEDDED_CLI11
+#include "CLI11.hpp"
+#else
+#include "CLI/CLI.hpp"
+#endif
 
 using namespace Mu;