Use CMake's FindIconv (see #10887)

[Croydon]

By the way, I have found that the Windows CI is always ending up using the lib binary files in git, even though that the CI is installing libiconv via another way. Example CI log: https://github.com/doxygen/doxygen/actions/runs/9178874312/job/25239722835#step:25:110. I am not going to change anything there as I consider it off-topic. Generally, I don't think that those built libraries should be in git and used. There should be enough options nowadays to get libiconv on Windows installed.

I renamed winbuild -> deps/iconv_winbuild to make it clearer what it is about. I had to give it sub-directories to let standard CMake's FindIconv find the right library.

See also #10887

[albert-github]

Regarding:

There should be enough options nowadays to get libiconv on Windows installed.

In GitHub there is indeed the step:

    - name: Install libiconv (Windows)
      uses: suisei-cn/actions-download-file@v1
      with:
        url: "https://github.com/pffang/libiconv-for-Windows/releases/download/v1.16/libiconv-for-Windows_1.16.7z"
        target: .
      if: matrix.config.os == 'windows-latest'

a bit strange that this is not used but I also don't see that the file is expanded.
@doxygen any idea why?

I tried to manually download the mentioned file but didn't succeed so I cannot tell whether the file would be suitable for usage in the GitHub Actions. I'm not even sure whether the file exists ...

@Croydon Do you have other suggestions for places to find an up to date libiconv (which can be used for "normal" / non CI usage) ?

@Croydon you used the the fix with the number of the related issue, due to the "magic" of GitHub adds a link to the issue (good) but it also adds a link that will automatically close the issue when this proposed Pull Request is integrated:

doxygen leaves the issues open and will automatically close the issue with a new release , please remove this automatic closing "merge" link.

[Croydon]

doxygen leaves the issues open and will automatically close the issue with a new release , please remove this automatic closing "merge" link.

Done. Thanks for pointing it out, have forgotten about it in the meantime.

I will comment on / look into the rest later.

[Croydon]

Just for protocol, I have a WIP branch to improve CI overall, but I unearthed several bugs in packages providing dependencies in the process. I will probably return to work on that eventually, but it might be months.

For now, everything merged so far should be fine for Doxygen, so it isn't really time-sensitive.