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

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

```cpp

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...

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