Fix incomplete/broken nghttp2 build support on Windows

[weltling]

Things done

• the flag name should be WITH_NGHTTP2, according to the current naming scheme. An external dep should always use "with" instead of "enable". The previous flag ENABLE_NGHTTP2 was kept for the compatibility reasons, so if passed it'll configure shared nghttp2 link as by default

• the meaning of "with" was extended to allow shared/static link of nghttp2, the corresponding flags are passed as required by nghttp2.

• nghttp2 does not depend on SSL of any kind, so the corresponding conditions are abandoned.

• the build folder name is extended with "nghttp2-<dll|static>", as it's done for other libs

• the library names are default as produced by the nghttp2 cmake configs.

thanks

[mention-bot]

@weltling, thanks for your PR! By analyzing the history of the files in this pull request, we identified @yangtse, @mback2k and @msnelling to be potential reviewers.

[rodwiddowson]

I'll happily review these and try to give them a spin if @welting can eyeball #1259 - otherwise it will never progress.

[weltling]

Thanks for keeping an eye, @rodwiddowson. This PR wasn't planned in the scope of #1322, but came out where i started the nghttp2 integration. I've just filed another one #1322, which targets the actual OpenSSL issue we have in a naive manner without breaking the current build approach. Basically, what were also discussed, is that one would keep the current approach, while making the lib names configurable. That's why I was wondering about #1259, as it seemed it were agreed to go by the suggestion to do it incrementally. We'll now need to choose a way to integrate these works somehow, but there will be for sure some merge conflicts. I would still suggest to go by an incremental integration, so we can thoroughly check every step and ensure the old way works, to hopefully get the whole accepted.

Thanks.

[bagder]

Thanks, squashed and merged!