deps: fix v8 valgrind warning

Fix the following valgrind warning:

    Conditional jump or move depends on uninitialised value(s)
        at 0x7D64E7: v8::internal::GlobalHandles::IterateAllRootsWithClassIds(v8::internal::ObjectVisitor*) (global-handles.cc:613)
        by 0x94DCDC: v8::internal::NativeObjectsExplorer::FillRetainedObjects() (profile-generator.cc:2849)
        # etc.

This was fixed upstream in r12903 and released in 3.15.2 but that commit
was never back-ported to the 3.14 branch that node.js v0.10 uses.

The code itself works okay; this commit simply shuffles the clauses in
an `if` statement to check that the node is in use before checking its
class id (which is uninitialized if the node is not in use.)
build: fix g++ 4.8 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.