Disable -Werror by default in nGraph build (#4296)
Disable -Werror by default in nGraph build (#4296)

This PR disables treating warnings as errors by default so that
nGraph can be built with compiler versions that are not currently
tested in CI. We could enable Werror in CI for those compiler versions
we want to target for warning-free builds.

Co-authored-by: Robert Kimball <f8f4fb0160c4e961c3d6afdbdcc40cfc21344ca8@intel.com>

NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph,NervanaSystems/ngraph