fix: Don't error when passing too new warning flags to old clang.

Older macOS versions with older xcode versions with older clang versions
don't have these warning flags. On newer ones, we do need those
suppressions. Alternative would be to detect support, but in this case,
we don't really care to be extremely clean.

Author: iphydf

qtox/build_qtbase_macos.sh

@@ -45,7 +45,7 @@ fi
 
 # We want -Werror to catch warnings related to macOS version compatibility.
 # We silence unused-variable because disabling features (like filesystemwatcher) can leave variables unused in Qt's source.
-sed -i '' -e 's/-Wextra/-Wextra -Werror "-Wno-#warnings" -Wno-cast-function-type-mismatch -Wno-deprecated-declarations -Wno-unused-but-set-variable -Wno-unused-variable -Wno-vla-cxx-extension/' cmake/QtCompilerFlags.cmake
+sed -i '' -e 's/-Wextra/-Wextra -Werror -Wno-unknown-warning-option "-Wno-#warnings" -Wno-cast-function-type-mismatch -Wno-deprecated-declarations -Wno-unused-but-set-variable -Wno-unused-variable -Wno-vla-cxx-extension/' cmake/QtCompilerFlags.cmake
 
 mkdir _build && pushd _build
 ../configure \