Add -DPYBIND11_WERROR=ON to mingw cmake commands

[rwgk]

Description

Add -DPYBIND11_WERROR=ON to mingw cmake commands, plus a couple minor tweaks to resolve warnings.

Brings the two MINGW jobs on par with all other CI jobs.

Suggested changelog entry:

[rwgk]

@henryiii this is ready for review now (I don't know if you get auto notifications, in addition to the auto-add as reviewer).

[rwgk]

@Skylion007 @henryiii did you see this PR?

I believe this is a very quick review. Just a little bit of tweaking to silence warnings about Eigen code, and about statics in a couple simple tests that are guarded from deletes only by runtime ifs.

I believe that extra little noise is a very small price to pay for having -DPYBIND11_WERROR=ON for mingw.

[rwgk]

@Skylion007
@henryiii

Hoping you can take a look. This is a very simple PR, plugging an obvious hole in our PYBIND11_WERROR coverage.

[henryiii]

Suggested change

	run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build2
	run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=14 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build2

[henryiii]

Suggested change

	run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build3
	run: cmake -G "MinGW Makefiles" -DCMAKE_CXX_STANDARD=17 -DCMAKE_VERBOSE_MAKEFILE=ON -DPYBIND11_WERROR=ON -DDOWNLOAD_CATCH=ON -S . -B build3

Let's be consistent, and this type is really only to help cmake-gui anyway.

[henryiii]

Minor nit, but otherwise seems fine. Verbose makefile make logs a bit long, not sure I prefer them, but okay if you want to.

[rwgk]

Thanks for the review!

Verbose makefile make logs a bit long, not sure I prefer them, but okay if you want to.

This setting is only for the CI. I often analyze the logs with scripts. Having the information in the logs makes the difference between day and night (seeing and being blind). Another way to look at this: reducing is easy and conclusive, guessing is hard and ambiguous.