provide private sqrt() or mandate -lm ?

### https://godbolt.org/z/EfEvq6nh3

Please see that Godbolt.

>  For well-known reasons linker complains and compilation is not done
>  linker complains `sqrt()` can not be found, thus '-lm' is required.

yes I know this is C only a Linux issue.  I added some `sqrt()` implementation and that code links ok, with no `-lm` linker requirement "drama".

This seems like an easy way out ...