autogen.sh: CFLAGS: Add some -Werror=... flags that will be default soon

autogen.sh: CFLAGS: Add some -Werror=... flags that will be default soon

Clang 16 and GCC 14 have upgraded several warnings to errors by default.
Also, there are new warnings that will be requirements of ISO C23.  Add
all of those to our build.

Use Clang's -Wno-unknown-attribute-option, to ignore warnings that are
exclusive of GCC.  Sadly, GCC doesn't have such an option.

Link: <https://wiki.gentoo.org/wiki/Modern_C_porting#What_changed.3F>
Link: <https://github.com/shadow-maint/shadow/issues/922>
Suggested-by: Sam James <sam@gentoo.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>