x11-toolkits/fox16: Genericize llvm16 fix from [1]

Checking against the operating system version doesn't make much sense
to me since the problem will return if not using the base compiler.

This change will also suppress warnings when building with older
versions of Clang.

[1] https://cgit.freebsd.org/ports/commit/?id=abad80143b961d4b2431114a4a5160d0efd04284