configure: Disable (clang) initializer-overrides warnings

Disable clang's initializer-overrides warnings, as QEMU makes significant
use of the pattern of initializing an array with a range-based default
entry like
    [0 ... 0x1ff] = { GPIO_NONE, 0 }
followed by specific entries which override that default, and clang
would otherwise warn "initializer overrides prior initialization of
this subobject" when it encountered the specific entry.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -1160,6 +1160,7 @@ gcc_flags="-Wold-style-declaration -Wold-style-definition -Wtype-limits"
 gcc_flags="-Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers $gcc_flags"
 gcc_flags="-Wmissing-include-dirs -Wempty-body -Wnested-externs $gcc_flags"
 gcc_flags="-fstack-protector-all -Wendif-labels $gcc_flags"
+gcc_flags="-Wno-initializer-overrides $gcc_flags"
 # Note that we do not add -Werror to gcc_flags here, because that would
 # enable it for all configure tests. If a configure test failed due
 # to -Werror this would just silently disable some features,