[fmt] Add usage file

[dgellow]

Describe the pull request

What does your PR fix?

When fmt is installed from vcpkg, you get the output:

The package fmt:x64-windows provides CMake targets:

    find_package(fmt CONFIG REQUIRED)
    target_link_libraries(main PRIVATE fmt::fmt fmt::fmt-header-only)

Unfortunately that's not correct, we shouldn't link both fmt::fmt and the header only version, fmt::fmt-header-only.

This PR adds a usage file to instead show to the user the CMake statements from fmt own documentation: https://fmt.dev/latest/usage.html#usage-with-cmake.

Which triplets are supported/not supported? Have you updated the CI baseline?

I don't think anything has to be updated. Just tell me if I have to change something.

Does your PR follow the maintainer guide?

I believe so, yes.

[PhoebeHui]

Could you also update the 'Port-Version: 2' to ''Port-Version: 3' in CONTROL file?

[ras0219-msft]

Hi @dgellow, thanks for the PR!

Did you verify that this actually achieves the change you wanted? The PR does not appear to deploy the usage file, so it will not affect the actual printed message in vcpkg.

The file needs to be deployed to share/fmt/usage.

[dgellow]

Thanks for the review. The usage file should be copied to the correct location now, though I have issues testing it locally because of network timeouts errors when fetching mingw from MSYS2. They seem to face some problems with their servers.

I will try again later, hopefully that will be fixed.

[BillyONeal]

Thanks for your contribution!