Add support for CMake 3.28 C++ module features

[jcar87]

CMake 3.28 (rc1 announcement) adds support for C++ modules that is no longer experimental (so no need to add the experimental flags).

This PR adds support for this using the new features, when the build is configured with FMT_MODULE set to ON and the CMake version is 3.28 or higher.

Tested both building fmt, and then calling cmake install and using the fmt module from a different project. This works with:

• Linux, Clang 17, ninja 1.11.1
• Linux, gcc 13.1, ninja 1.11.1 (note that I'm using a patched version of gcc with dependency scanning support, and additional CMake variables since CMake expects gcc 14 for this, but gcc14 ICEs when building fmt as a module, I've reported this here)
• Windows, msvc 19.37
  • Building fmt itself works with the Visual Studio generator or Ninja
  • Using the imported targets with find_package(fmt) works with Ninja, but not with the Visual Studio generator - reported this here.

Hopefully this is OK. For CMake versions earlier than 3.28, this should behave as it was previously.

[vitaut]

Thanks for the PR but the part of CMake you are changing is deprecated and will be removed. Please add this to add_module_library instead.

[jcar87]

Hi @vitaut - thanks for the review.
The changes in this PR are mostly in add_module_library() - the only change in enable_module() is that I've added a conditional to skip the logic altogether when CMake is >= 3.28, as I can see that enable_module() is still called, and what it aims to do is already handled by CMake+ninja or by MSBuild.

However if this is not the best way, I'm happy to move the conditional out of enable_module() and into the two places where it is still called?

[vitaut]

Sorry, I just realized that it's not super clear from the code and my comment but what I meant is that the source of truth of add_module_library is in https://github.com/vitaut/modules. Could you submit the change there to prevent it being overwritten in this repo next time we sync the two?

[vitaut]

I merged it for now but as commented earlier it will be overwritten unless also added to the main modules repo. Thanks!

[vitaut]

Revered because of incompatibility with CMake 3.16: #3714.

[jcar87]

Apologies @vitaut - I will make the contribution in the main modules repo and make sure there's no regression with older CMake.

[vitaut]

No worries and thanks for working on this.