Meson: use system CLI11 if available, fallback on subproject

[marcusmueller]

I'm trying to automate newsched package building, but to adhere to distro packaging guidelines, I mustn't use subprojects for available libraries; so I followed https://mesonbuild.com/Subprojects.html#toggling-between-system-libraries-and-embedded-sources in hopes this will lead to sufficiently modern CLI11.

[jsallay]

This looks entirely reasonable.