Add .with_rustfmt("rustfmt") if the which-rustfmt feature is disabled?

[heidezomp]

I was very pleased to see that #1615 reduces the number of dependencies used by bindgen. However, now my bindings don't get formatted unless I add .with_rustfmt("rustfmt") to the builder options. Since the which dependency resolves a relative path to an absolute path, and adding the .with_rustfmt("rustfmt") option has the same effect but with the shell doing the resolving, would this be a reasonable fallback to add in case the which-rustfmt feature is disabled?

[emilio]

Perhaps, that would make sense to me.

[heidezomp]

Thanks for the quick reply and merge! ❤️