LIEF is built with two C++ standard version flags

[targos]

See in https://ci.nodejs.org/job/node-test-commit-linux/68776/nodes=fedora-latest-x64/consoleFull

LIEF lines have both: -std=gnu++20 -std=gnu++17

[joyeecheung]

Where does -std=gnu++20 come from? I think for LIEF we can specifically cflags_cc! that, LIEF currently needs to be built with C++17 to avoid namespace conflicts.