build: only define NODE_V8_OPTIONS if not empty
build: only define NODE_V8_OPTIONS if not empty

Previously, V8::SetFlagsFromString was called on every launch even if
NODE_V8_OPTIONS was an empty string. This patch only defines
NODE_V8_OPTIONS if node_v8_options is not an empty string.

PR-URL: https://github.com/nodejs/node/pull/2532
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Rod Vagg <f9f03903429036ef609ed7ca822fddfa76155c29@vagg.org>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node