Fix gyp_v8 to work with use_system_icu=1

If you want to use the system icu library, you'll need to use gyp_v8 (as
opposed to the standalone Makefiles)

BUG=v8:2475
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/19195003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15677 ce2b1a6d-e550-0410-aec6-3dcde31c8c00