[build2] fmt library module

[Lucas Ransan]

Boris Kolpackov <boris at codesynthesis.com> writes:

> You also need to import lib{fmt} in your buildfile and link it to your
> exe{}.

Yes, I did that too.

> Generally, I would recommend to first make sure everything works
> with headers and then try to switch to modules.

It works when including fmt headers, but then I can't use system modules
(like import <iostream>;) or else I get linking errors.

> So, in summary, I think if you would like to start using fmt as
> a module and expect a smooth experience, then unfortunately you
> will be disappointed, probably still for some time to come.

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.

> But if you are keen to help hack it into shape, we can give it a try.

I would like to help, but I don't think I have enough knowledge in C++ and build2.

Anyway, thank you for your insight!