build: unconditionally disable -Werror

Forcibly disable -Werror, the old { 'werror': '' } hack in node.gyp
no longer works with newer versions of V8.

We support a wide range of compilers, it's simply not feasible to
squelch all warnings, never mind that the libraries in deps/ are
not under our control.

Fixes #6817.

Suggested reviewer(s): @tjfontaine @trevnorris