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 embeds a "vendored" subset of the libfmt package 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-fmt=true is
specified.