Disabling -Wsuggest-override inside macros is broken in gcc, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55578, and has started
affecting wxWARNING_SUPPRESS_MISSING_OVERRIDE since gcc 11, i.e. this
macro doesn't have any effect any more and the warning is still given.
Avoid it by actually specifying "override" for gcc 11 (as doing it for
all compilers would result in -Winconsistent-missing-override from
clang) and check that we don't get this warning in the allheaders test.
Also don't use wxDECLARE_ABSTRACT_CLASS() inside wxObject itself, now
that it uses "override", which is not appropriate for the base class
version. This is arguably more clear and should have been done like this
since the beginning anyhow.
This warning flag is really only used for stdlib debugging/writing,
and is off by default so that normal users of the library don't see
any warnings generated by the library (since they have no control
over it).
We don't use "final" in our code, as very few classes in wx code have
virtual functions but are not meant to be derived from in the user code.
Ideal would be to check the existing warnings and maybe apply "final" if
it's relevant and disable it otherwise, as these warnings can be useful
to build the application code with, but for now just disable them in the
test suite.
As wx headers are included from user applications which may compile with
higher warning level than wx itself, try to check headers compilation
with almost all of gcc warning flags turned on.
This notably should prevent the headers from becoming uncompilable with
-pedantic again in the future.
Check that all our public headers compile with this macro defined and
that using a char string without specifying its expected encoding
results in the expected compilation failure in this case.
