Add pkgconfig file for fmt-c library

[Ferdi265]

This PR adds a pkgconfig file for the fmt-c library, allowing the C API to be found and linked via pkgconfig. The CMake package already exports fmt::fmt-c, so that doesn't need changes.

The only thing I was unsure of was if fmt-c.pc should explicitly link -lfmt as well, or if the implicit transitive linkage is enough. For now I have added it, since the fmt::fmt-c CMake target explicitly lists fmt::fmt as a linked library as well, but it can be removed if not needed.

Also, currently the fmt::fmt-c target is part of the fmt_core CMake package component, so I added the fmt_c.pc file to fmt_core as well. Maybe that could be changed to a fmt_c or fmt_c_api component.

[vitaut]

Thanks for the PR.

fmt is an implementation dependency of fmt-c, so I think this should use Requires.private: fmt = @FMT_VERSION@ rather than putting -lfmt in Libs.

[Ferdi265]

Thanks for the PR.

fmt is an implementation dependency of fmt-c, so I think this should use Requires.private: fmt = @FMT_VERSION@ rather than putting -lfmt in Libs.

I didn't know about Requires.private. Will use that instead, thanks!

[Ferdi265]

Added Requires.private instead of directly linking -lfmt.

[vitaut]

Merged, thanks