configure: Bump minimum Clang version to 10.0

configure: Bump minimum Clang version to 10.0

Anthony Perard recently reported some problems with Clang v6.0 from
Ubuntu Bionic (with regards to the -Wmissing-braces configure test).
Since we're not officially supporting that version of Ubuntu anymore,
we should better bump our minimum version check in the configure script
instead of using our time to fix problems of unsupported compilers.
According to repology.org, our supported distros ship these versions
of Clang (looking at the highest version only):

              Fedora 36: 14.0.5
      CentOS 8 (RHEL-8): 12.0.1
              Debian 11: 13.0.1
     OpenSUSE Leap 15.4: 13.0.1
       Ubuntu LTS 20.04: 12.0.0
          FreeBSD Ports: 15.0.7
          NetBSD pkgsrc: 15.0.7
               Homebrew: 15.0.7
            MSYS2 mingw: 15.0.7
            Haiku ports: 12.0.1

While it seems like we could update to v12.0.0 from that point of view,
the default version on Ubuntu 20.04 is still v10.0, and we use that for
our CI tests based via the tests/docker/dockerfiles/ubuntu2004.docker
file.

Thus let's make v10.0 our minimum version now (which corresponds to
Apple Clang version v12.0). The -Wmissing-braces check can then be
removed, too, since both our minimum GCC and our minimum Clang version
now handle this correctly.

Message-Id: <20230131180239.1582302-1-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>