Incompatible change: The default for use_cpp_toolchain to be mandatory.

[comius]

The default value of mandatory changed from False to True https://github.com/bazelbuild/rules_cc/commit/07e3cb66d0d516d5d855c745a3a1b5bae0b67e5e

First affected release is rules_cc 0.1.2

The default behaviour should be better in the most of the cases.