Add Cygwin CI and stop installing unwanted plugins

[DWesl]

This adds a CI job for my development platform, which will hopefully avoid issues like #2196 and #2446.

This also avoids installing test plugins (or any plugins if --without-plugin-dir is passed to configure), which is especially noticed by distribution package managers using a DESTDIR install: that issue was previously raised as #2431.

I tried to get CMake CI runs working on Cygwin and MinGW, but couldn't get any tests to pass on Cygwin or complete the build on MinGW; this is likely due to the problems highlighted in #2290.

Includes cross-compilation suggestion from this comment.

Also includes a few markdown cleanups that seem to have been fixed in other ways since I started working; let me know if you want to approach those changes differently

Replaces #2431; replaces #2440 Might close #2468 Might replace #2479 Might close #2396

[CLAassistant]

All committers have signed the CLA.

[DennisHeimbigner]

This PR will take some time to digest, but thanks for doing this.

[DennisHeimbigner]

OK, I think this is ok to merge subject to testing with Visual Studio. [Update tested with visual studio (with filters off) and all tests passed]

[DWesl]

Do you want me to try to add MSVC testing to CI as part of this PR? CMake should be straightforward; Autotools will need help (specifically, someone telling me what values to specify for CC, CPP, and LD)

[DennisHeimbigner]

I do note that the Mac build is currently failing for filters.

[DWesl]

It's passing in #2527 just before the merge, and on #2519, which is the most likely place to have broken things. Given this comment, the only thing I can think of is that I added AM_LDFLAGS to each plugins' _LDFLAGS and maybe -avoid-version doesn't play nicely with -version-info 0:0:0. Given that every plugin now specifies -version-info, I'll replace -avoid-version in AM_LDFLAGS with -version-info

[DWesl]

Apparently -version-info is required for plugins on MacOS and breaks plugins on MinGW and Cygwin, which is not what I expected

[WardF]

Reviewing extant PR's to pull into v4.9.1-rc2; thanks a lot, I just noticed the checks hadn't run. Running now.

[WardF]

@DennisHeimbigner checks are passing, but you are more plugged into the cygwin ecosystem. Any blocking issues I'm not seeing, here? Otherwise I'll merge it later this afternoon :).

[DennisHeimbigner]

I think my previous review holds, so it should be ok to merge it.

[WardF]

Thanks @DennisHeimbigner !