build: Disable noisy `-Wreturn-type` for MinGW-w64 #26190

On the master (9fcdb9f3a044330d3d7515fa35709102c98534d2) we have -Wno-error=return-type for MinGW-w64, to allow us successfully cross-compile Windows release binaries which also being configured with --enable-werror.

Although, even during daily cross-compiling (which is usual for me) such warnings are noisy, distracting and useless, as all of them are false-positive. Disabling them for MinGW-w64 looks safe now because code quality is checked by other compilers, including MSVC. Also, after moving to std::filesystem, we have much less Windows-specific code paths.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #25972 (build: no-longer disable WARN_CXXFLAGS when CXXFLAGS is set by fanquake)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

~0. I think this should just be removed entirely, and if someone wants to suppress it (regardless of if they are building with -Werror), they can add -Wno-return-type to their CXXFLAGS.

In any case, you can't make the change (in it's current form) until after 25972. Otherwise the CXXFLAGS being dropped will cause the CI to fail. (another reason to just add it to CXXFLAGS when required, including in the CI).

In any case, you can't make the change (in it's current form) until after 25972. Otherwise the CXXFLAGS being dropped will cause the CI to fail. (another reason to just add it to CXXFLAGS when required, including in the CI).

Another option is to -Wno-return-type in depends/hosts/mingw32.mk?

if someone wants to suppress it (regardless of if they are building with -Werror), they can add -Wno-return-type to their CXXFLAGS.

For regular builds it is a good option. But for working on the build system itself it looks suboptimal...

Another option is to -Wno-return-type in depends/hosts/mingw32.mk?

NACK.

For regular builds it is a good option. But for working on the build system itself it looks suboptimal...

Why?

For regular builds it is a good option. But for working on the build system itself it looks suboptimal...

Why?

To distinguish effect of compiler flags provided in different ways, it is useful to have a clean build with no user-provided flags (mostly related to my work on CMake-based system).

I think this should just be removed entirely

Done in #26504.