Howto BuildRequire a higher gcc version for #include <filesystem>? Or else?

Hello, I am trying to build a c++ based package on obs for 15.4, 15.5, Tumbleweed, which includes <filesystem>. Unfortunately this seems to be standard from gcc-c++>=11 on, only. BuildRequires: gcc>=11 results in "nothing provides gcc>=11". I have also tried "gcc >=11", and BuildConflicts: gcc however, this completely breaks. I have tried "gcc11-c++" which installs, but is not used in the build. Is it possible to switch to a higher gcc version via spec file alone? Or is it possible to compile this with gcc<11 by means of some CC flags? I have searched on the net extensively, without result. I would appreciate a pointer to any information that could help. Thank you in advance, Alex