Fixed gRPC compilation with Clang when building Mesos.
Fixed gRPC compilation with Clang when building Mesos.

When compiling gRPC with Clang, there are some array-out-of-bound
warnings due to the use of GLIBC's `__strcmp_cg` macro in the c-ares
library. With `-Werror` on, these warnings would stop gRPC from
compiling. This patch ignores such errors.

Review: https://reviews.apache.org/r/68114

jpeach/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,chhsia0/mesos,Gilbert88/mesos,chhsia0/mesos,andschwa/mesos,kaysoky/mesos,abudnik/mesos,gsantovena/mesos,Gilbert88/mesos,andschwa/mesos,reneploetz/mesos,reneploetz/mesos,jpeach/mesos,abudnik/mesos,jpeach/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,andschwa/mesos,andschwa/mesos,reneploetz/mesos,gsantovena/mesos,Gilbert88/mesos,dforsyth/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,gsantovena/mesos,dforsyth/mesos,andschwa/mesos,reneploetz/mesos,jpeach/mesos,reneploetz/mesos,chhsia0/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,kaysoky/mesos,gsantovena/mesos,Gilbert88/mesos,dforsyth/mesos,abudnik/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,jpeach/mesos,kaysoky/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,chhsia0/mesos,kaysoky/mesos,dforsyth/mesos