[libiconv] Build on macOS

[ChaseKnowlden]

Made with Google Antigravity

Use GNU libiconv instead of the Xcode shipped libiconv due to issues with the Xcode shipped libiconv.

Related #52232

If this PR updates an existing port, please uncomment and fill out this checklist:

• Changes comply with the maintainer guide.
• SHA512s are updated for each updated download.
• The "supports" clause reflects platforms that may be fixed by this new version, or no changes were necessary.
• Any fixed CI baseline and CI feature baseline entries are removed from that file, or no entries needed to be changed.
• All patch files in the port are applied and succeed.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Exactly one version is added in each modified versions file.