GCC should use gnu17

[pehrsoderman]

No description provided.

[mpsijm]

Thanks for your recent updates to languages.yaml! 😄 Just to double-check, https://open.kattis.com/languages/c still says -std=gnu23, is that intentional?

(This may be a similar thing as what I mentioned in #257 (comment), where the Kotlin version at https://open.kattis.com/languages/kotlin is still at 1.8, even though Kattis internals use(d?) 1.3)