* Add `-DPYBIND11_WERROR=ON` to mingw cmake commands (and `-DCMAKE_VERBOSE_MAKEFILE:BOOL=ON`).

* Using no-destructor idiom to side-step overzealous MINGW warning.

* Add __MINGW32__ pragma GCC diagnostic ignored in eigen.h

* Add another no-destructor workaround.

* Temporarily add -k (keep-going) flags to hopefully speed up finding all warnings.

* Revert "Temporarily add -k (keep-going) flags to hopefully speed up finding all warnings."

This reverts commit f36b0af8f93e9f6f16c969fb5b646e116f5eaf0f.

* Very minor shuffle to avoid MSVC warnings.

* Remove all `:BOOL` as suggested by @henryiii