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.

meson_options.txt

@@ -25,6 +25,11 @@ option('use-embedded-fmt',
        value: false,
        description: 'Use the embedded fmt, even if system package is found')
 
+option('use-embedded-cli11',
+       type: 'boolean',
+       value: false,
+       description: 'Use the embedded CLI11, even if system package is found')
+
 #
 # language detection
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