Add automatic DLL support for Windows Ruby Installer

[johnnyshields]

This PR uses the Gemspec msys2_mingw_dependencies metadata to automatically install the appropriate Mingw package. You can see the same thing is used for the Psych gem here.

This saves Windows users the trouble of downloading/renaming the DLL file.

You can see here the file is located at /ucrt64/bin/libsodium-26.dll on the Mingw package, hence we need to add a new possible package name to the ffi_lib call.

[johnnyshields]

@tarcieri can you please take a look at this?

[tarcieri]

@johnnyshields can you rebase?