CompilerWarnings task does not catch C++ warnings

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.