Remove WARNFLAGS

[Algunenano]

As with #4629, this flags are not used in multiple folders since they are not standard.

I suggest dropping them completely, whoever is building the projects is free to add them to their CFLAGS if they want. They are only useful in CI if they are matched with -Werror to force the build to fail as otherwise they are not checked.

OTOH, if we wanted to keep them I'd rather move the output to add them to the CPPFLAGS so they are included everywhere, not just where we remembered to add WARNFLAGS manually.

[Raúl Marín <git@…>]

In cdd520d3/git:

Remove WARNFLAGS

Closes #4630

[strk]

I actually missed the -Wall. First quality check should be done by developers, so I disagree that's ONLY useful for CI. Please see #5002