Include directories should be set to SYSTEM if not stand-alone

[Krzmbrzl]

If CLI11 is not built as a standalone library, but instead as part of another project (e.g. via submodule or via cmake's FetchContent) target_include_directories should make use of the SYSTEM option. This will have the effect that the compiler will not emit any warnings for these include files, even if included in files for which the warning settings would normally cause a warning to be emitted.

So as a trivial example, if in your header you somewhere had

unsigned int i = -1;

and I compile my project with -Wsign-conversion, I get a warning for this line in your header, if I include that in my source file (for which that warning applies). Marking the CLI11 headers as "system headers" will make the compiler silently ignore that warning - which seems to be what downstream users want...

[Krzmbrzl]

Note: If you agree that this is desirable, I could create a PR for this

[phlptp]

I think this would be of interest.