is_flag_supported returns false negatives

[RubixDev]

Seems like #892 is back. I once submitted a patch in #839 and that worked for me at the time, but after an update to cc v1.2.15 I'm having the same issues again. I'm on wasm32-unknown-unknown and using .flag_if_supported("-Wno-unused-parameter") doesn't add the flag even though it's supported.

[NobodyXu]

Hmmm is looks like the flag is removed.

[ActuallyHappening]

Wow I'm glad this was one of the top issues, I've come across a false negative here: https://github.com/brson/wasm-opt-rs/blob/8854bd8a58d7393c7bacd950884aea521ce8e19f/components/wasm-opt-sys/build.rs#L8
I'm unable to build wasm-opt on a new NixOS system because it complains my clang --version 20.1.1 compiler C++ compiler does not support -std=c++17 flag, which originates here: https://github.com/brson/wasm-opt-rs/blob/8854bd8a58d7393c7bacd950884aea521ce8e19f/components/wasm-opt-sys/build.rs#L367

[NobodyXu]

I have opened PR #1424 but I don't have time to fix the CI errors, if anyone is willing to pickup and fix it, I'm happy to merge it