cppunit: disable blanket -Werror
cppunit: disable blanket -Werror

`-Werror` flag usually causes build failures due to minor changes in
compiler versions. They might be useful for developers themselves but
are rarely useful for distributions.

For example right now `cppunit` fails to compile on `gcc-13` due to a
`gcc` infelicity: https://gcc.gnu.org/PR107488

While this concrete instance is a compiler bug generally `-Werror` makes
users' lives harder. Specific `-Werror=<foo>` are better way to prevent
certain classes of bugs.

The change removes planket `-Werror` with `--disable-werror` flag.

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