[Bug libstdc++/89090] vector.tcc uses "if constexpr" in C++11 mode

[csaba_22 at yahoo dot co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89090

--- Comment #7 from Csaba Ráduly <csaba_22 at yahoo dot co.uk> ---
Don't be sad, use -std=c++17 :)