解决办法：defined but not used [-Werror=unused-variable]

 遇到错误如下：

/grpc/gens/src/proto/grpc/core/stats.pb.cc:187:13: error: ‘dynamic_init_dummy_src_2fproto_2fgrpc_2fcore_2fstats_2eproto’ defined but not used [-Werror=unused-variable]
 static bool dynamic_init_dummy_src_2fproto_2fgrpc_2fcore_2fstats_2eproto = []()
             ^
cc1plus: all warnings being treated as errors

找到Makefile，去掉其中-Werror ，重新编译。