CMakeLists.txt: pass Clang-specific warning flag to Clang only
CMakeLists.txt: pass Clang-specific warning flag to Clang only

SeReRPRo/badvpn,SeReRPRo/badvpn,SeReRPRo/badvpn,SeReRPRo/badvpn,SeReRPRo/badvpn