build: disable -Werror

Turn off -Werror when building V8, it hits -Werror=unused-local-typedefs
with g++ 4.8.  The warning itself is harmless so don't abort the build.

This was originally implemented in commit d2ab314e back in 2011 but the
build process has gone through a few iterations since then, that change
no longer works.

Suggested reviewer: @tjfontaine for no particular reason.