Use icu_gyp_path variable to reference ICU gyp file

That way, embedders can put the icu sources whereever they want and
aren't forced to put them under third_party/gyp

BUG=none
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/23598011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16749 ce2b1a6d-e550-0410-aec6-3dcde31c8c00