devel/ocaml-findlib: the port had been updated to version 1.8.1

• This is not the latest version, but this step lubricates coming next phase of our OCaml stack update
• Since we're post-4.06 now, remove support for Num library which is a separate port/package
• Detect optionally installed libraries and adjust the packaging list accordingly
• Spell `Tk' correctly in the TOOLBOX option description

PR: 238904, 262791