[build2] fmt library module

[Boris Kolpackov]

Lucas Ransan via users <users at build2.org> writes:

> I was hoping I could use fmt instead of <format>, since it is not
> supported by GCC for now.
> I'm really surprised that C++ features take so much time to implement,
> considering it's a widely used language.

I am not sure what's holding up <format> support in GCC (perhaps
the availability of this feature as a library makes GCC developers
prioritize other features). As for modules, this is an unprecedented
change to the C++ language and its compilation model and I don't
think we should be particularly surprised it's taking a while to
implement.