cmake: silence -Wsign-compare in libfmt

cmake: silence -Wsign-compare in libfmt

* silence -Wsign-compare in libfmt
* silence -Wattributes in seastar

to silence warnings like:

/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/fmt/include/fmt/format.h:
In instantiation of 'constexpr bool fmt::v5::internal::find(Ptr, Ptr, T,
Ptr&) [with bool IS_CONSTEXPR = false; T = char; Ptr = const wchar_t*]':
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/fmt/include/fmt/format.h:2154:32:
required from 'constexpr void
fmt::v5::internal::parse_format_string(fmt::v5::basic_string_view<Char>,
Handler&&)::writer::operator()(const Char*, const Char*) [with bool
IS_CONSTEXPR = false; Char = wchar_t; Handler =
fmt::v5::format_handler<fmt::v5::arg_formatter<fmt::v5::back_insert_range<fmt::v5::internal::basic_buffer<wchar_t>
> >, wchar_t,
fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<wchar_t>
>, wchar_t> >&]'
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/fmt/include/fmt/format.h:2164:3:
required from 'constexpr void
fmt::v5::internal::parse_format_string(fmt::v5::basic_string_view<Char>,
Handler&&) [with bool IS_CONSTEXPR = false; Char = wchar_t; Handler =
fmt::v5::format_handler<fmt::v5::arg_formatter<fmt::v5::back_insert_range<fmt::v5::internal::basic_buffer<wchar_t>
> >, wchar_t,
fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<wchar_t>
>, wchar_t> >&]'
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/fmt/include/fmt/format.h:3346:39:
required from 'typename Context::iterator fmt::v5::vformat_to(typename
ArgFormatter::range, fmt::v5::basic_string_view<Char>,
fmt::v5::basic_format_args<Context>) [with ArgFormatter =
fmt::v5::arg_formatter<fmt::v5::back_insert_range<fmt::v5::internal::basic_buffer<wchar_t>
> >; Char = wchar_t; Context =
fmt::v5::basic_format_context<std::back_insert_iterator<fmt::v5::internal::basic_buffer<wchar_t>
>, wchar_t>; typename Context::iterator =
std::back_insert_iterator<fmt::v5::internal::basic_buffer<wchar_t> >;
typename ArgFormatter::range =
fmt::v5::back_insert_range<fmt::v5::internal::basic_buffer<wchar_t> >]'
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/fmt/include/fmt/format.h:3457:64:
required from here

/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/fmt/include/fmt/format.h:2120:14:
error: comparison of integer expressions of different signedness: 'const
wchar_t' and 'char' [-Werror=sign-compare]
     if (*out == value)
         ~~~~~^~~~~~~~
cc1plus: all warnings being treated as errors

where libfmt compares a wchar_t with a literal '}', which is char.
because the former is unsigned, and the latter is of a signed type,
GCC is annoyed. but since both of them are ASCII, and when performing
comparison, the signed one is converted to unsigned, then result of
comparison is correct per-se. hence, it's safe to silence this very
warning.

and warning like:

/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/future-util.cc:61:5:
required from 'seastar::future<> seastar::sleep_abortable(typename
Clock::duration, seastar::abort_source&) [with Clock =
std::chrono::_V2::steady_clock; typename Clock::duration =
std::chrono::duration<long int, std::ratio<1, 1000000000> >]'
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/future-util.cc:68:105:
required from here
/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/future-util.cc:48:28:
error: 'seastar::sleep_abortable(typename Clock::duration,
seastar::abort_source&)::sleeper::sleeper(typename Clock::duration,
seastar::abort_source&) [with Clock = std::chrono::_V2::steady_clock;
typename Clock::duration = std::chrono::duration<long int, std::ratio<1,
1000000000> >]::<lambda()>' declared with greater visibility than the
type of its field 'seastar::sleep_abortable(typename Clock::duration,
seastar::abort_source&)::sleeper::sleeper(typename Clock::duration,
seastar::abort_source&) [with Clock = std::chrono::_V2::steady_clock;
typename Clock::duration = std::chrono::duration<long int, std::ratio<1,
1000000000> >]::<lambda()>::<this capture>' [-Werror=attributes]
                 : tmr([this] { done.set_value(); }) {
                            ^
Signed-off-by: Kefu Chai <kchai@redhat.com>