Build: Detect supported compiler warning flags better.

Clang and nowadays also GCC accept any -Wfoobar option
but then may give a warning that an unknown warning option
was specified. To avoid adding unsupported warning options,
the options are now tested with -Werror.

Thanks to Charles Diza.
l10n: vi.po: Update Vietnamese translation

Signed-off-by: Trần Ngọc Quân <vnwildman@gmail.com>