For the record, in the beginning Bazel does not support using auto-configured system compiler at all. The only thing it supports is using a controlled build environment and hardcode all paths (including compiler paths), or using a checked-in toolchain. But at a later stage a dumb autoconf-y "detect system toolchain and try to use it" thing was added and it became default.

This is because configuring a checked-in toolchain in Bazel needs tremendous effort and very specific knowledge about something 99.9% of developers does not know and DOES NOT WANT to know or spend their time on.