CheckCompilerFlag: Handle -Werror= for GCC 10+
GCC 10 and above warn about warning flags for other languages differently than error flags for different languages:
$ g++ -Wprotocol -Werror=protocol x.cpp
cc1plus: warning: command line option '-Wprotocol' is valid for ObjC/ObjC++ but not for C++
cc1plus: warning: '-Werror=' argument '-Werror=protocol' is not valid for C++
Since check_compiler_flag
thinks the first warning means that a flag is not accepted, it should also think so for the second warning.
Backport: release