[Feature Request] CMake option to use system-provided yyjson

[FedericoSchonborn]

Current state:

yyjson is downloaded during the build step (and vendored as of cd4a512), and there's no option to use the system-provided version of the library. Gentoo has a patch that removes the download step to use the system-provided yyjson instead.

Wanted state:

An option to use the system-provided yyjson should be available in CMake or the download/vendoring should be disabled if a system-provided version of yyjson is found.

Why the change is sensible:

Many Linux distributions would benefit from this feature, as they usually want to avoid enabling networking during builds and vendored libraries.

[CarterLi]

Hello.

Fastfetch v2.0.1 has been released, with -DENABLE_SYSTEM_YYJSON support. Please check it.

Also please check if the terminal detection is correctly fixed.