The CompilerWarnings task on Cirrus CI does not catch warnings in C++
code. It tries to make warnings fatal by passing COPT='-Werror', but
that does not apply to C++ compilations.
I suggest that we just add COPT to CXXFLAGS as well. I think passing
-Werror is just about the only reasonable use of COPT nowadays, so
making that more robust seems useful. I don't think there is a need for
a separate make variable for C++ here.