Detect system libcxx name with the CMake build system and convey that
information to build.zig via config.h so that the same system libcxx is
used for stage3.
This undoes the hacky search 901457d173467c71b681a8c69f4b77c94d516da7 .
closes #17018
3cf71580c4Michael Dusan committed on 9/3/2023, 6:38:03 AM· committed by Jakub Konkaparent1816bb4