fix nokogumbo linking on windows

[flavorjones]

What problem is this PR intended to solve?

Closes #2167 which relates to an issue building Nokogumbo on Windows against the precompiled libraries.

This PR does the following:

• ensures that libxml2 symbols are exported when built on windows
• ensures that nokogiri symbols are exported when built on windows
• includes LDFLAGS in Nokogiri::VERSION_INFO to allow the windows linker to resolve all symbols

Have you included adequate test coverage?

There's pretty good test coverage of this test case in the Nokogumbo CI suite; and I've added some light tests here in scripts/test-gem-installation though I'll note that we're not testing gem installation on Windows in the Nokogiri CI suite at the moment (though, see #2153).

Does this change affect the behavior of either the C or the Java implementations?

This should only impact downstream gems like Nokogumbo who are trying to link against the precompiled Nokogiri libraries.

[codeclimate]

Code Climate has analyzed commit 3cff820 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 33.3% (80% is the threshold).

This pull request will bring the total coverage in the repository to 94.0% (-0.2% change).

View more on Code Climate.

[larskanis]

@flavorjones I'm sad, I wasn't able to help you, but I'm glad, you could solve this issue!

I like the idea of passing cflags and ldflags by the VERSION_INFO. And the proposed solution makes sense to me. I only wonder why it worked with previous nokogiri versions. And I'm pretty sure, this will not work with MSWIN alias MSVC compilers.

Unfortunately I'm still suffered by pandemic restrictions. Schools are closed since months and working as a substitute teacher while having a lot of work on the daily job is exhausting. I have to take my holiday to do any serious OSS development.

[flavorjones]

@larskanis I don't think it ever worked on Windows ... previous versions of Nokogiri didn't package the header files as expected/promised, and so Nokogumbo will gracefully "fall back" to using Nokogiri's Ruby API (rather than libxml2's C API).