Enabled `--enabled-bundled-ffi` option for only mswin environment

[hsbt]

We discussed at https://bugs.ruby-lang.org/issues/18034

[eregon]

I'm unsure if we should prevent it on Linux. There I think using the bundled libffi usually works fine. Is libffi already a dependency to build CRuby? ruby-build at least includes it.

cc @LarsKanis you probably know more about this subject than I do, and the FFI gem tests both IIRC.

[larskanis]

ffi.gem bundles libffi since more than 10 years. The extconf.rb file checks whether system libffi is usable and otherwise falls back to builtin. Since commit https://github.com/ffi/ffi/commit/f0959778cfac41e4673117993c3abf26ef1ae76c it's also possible to force system or builtin libffi, so that users with an issue with the default selection can switch to the other way.

libffi on Macos is a mess. We had several bug reports which showed, that that header files don't fit to the libraries or that very old libffi versions were installed as system libffi. We did some fine tuning and now the reports are quite rare. Or maybe newer Macos versions fixed these issues or just stackoverflow has enough answers in the meantime.

Nevertheless builtin libffi works just fine on most operation systems and we test this on Windows, Linux and Macos: https://github.com/ffi/ffi/actions/runs/1287629846

[hsbt]

I and @unak try to use libffi provided by vcpkg. After that, We can remove the bundled source of libffi for mswin environment.

[kou]

@larskanis @hsbt Thanks for the information. I'll consider libffi bundling again in this month.

[hsbt]

bundled source has been removed at https://github.com/ruby/ruby/commit/e4f5296f065110fa83eb450d3a861253e76e534f

[kou]

Let's remove bundled libffi in ruby/fiddle too.

[hsbt]

I'm going to submit another PR for removing download feature.