configure: bump min required CLang to 6.0 / XCode 10.0

Several distros have been dropped since the last time we bumped the
minimum required CLang version.

Per repology, currently shipping versions are:

             RHEL-8: 10.0.1
      Debian Buster: 7.0.1
 openSUSE Leap 15.2: 9.0.1
   Ubuntu LTS 18.04: 6.0.0
   Ubuntu LTS 20.04: 10.0.0
         FreeBSD 12: 8.0.1
          Fedora 33: 11.0.0
          Fedora 34: 11.1.0

With this list Ubuntu LTS 18.04 is the constraint at 6.0.0

An LLVM version of 6.0.0 corresponds to macOS XCode version of 10.0
which dates from Sept 2018.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210514120415.1368922-13-berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -2056,12 +2056,12 @@ fi
 cat > $TMPC << EOF
 #if defined(__clang_major__) && defined(__clang_minor__)
 # ifdef __apple_build_version__
-#  if __clang_major__ < 5 || (__clang_major__ == 5 && __clang_minor__ < 1)
-#   error You need at least XCode Clang v5.1 to compile QEMU
+#  if __clang_major__ < 10 || (__clang_major__ == 10 && __clang_minor__ < 0)
+#   error You need at least XCode Clang v10.0 to compile QEMU
 #  endif
 # else
-#  if __clang_major__ < 3 || (__clang_major__ == 3 && __clang_minor__ < 4)
-#   error You need at least Clang v3.4 to compile QEMU
+#  if __clang_major__ < 6 || (__clang_major__ == 6 && __clang_minor__ < 0)
+#   error You need at least Clang v6.0 to compile QEMU
 #  endif
 # endif
 #elif defined(__GNUC__) && defined(__GNUC_MINOR__)
@@ -2074,7 +2074,7 @@ cat > $TMPC << EOF
 int main (void) { return 0; }
 EOF
 if ! compile_prog "" "" ; then
-    error_exit "You need at least GCC v7.5 or Clang v3.4 (or XCode Clang v5.1)"
+    error_exit "You need at least GCC v7.5 or Clang v6.0 (or XCode Clang v10.0)"
 fi
 
 # Accumulate -Wfoo and -Wno-bar separately.