bump to C++17
Currently, piglit is at gnu99
and since the even the Linux kernel itself moved to gnu11
, we could follow Mesa3D and bump to C++17, which would allow developers to use a bit more modern C++.
While my reasons are weak (having [[maybe_unused]]), I'm just dropping this here, as it could improve developers' workflow.
C++17 has been available since GCC 8+ and LLVM 5.0+, which are currently present even in old-stable Debian.
Maybe it's the time?