Add use_system_v8 option to gyp (off by default),

as discussed in http://groups.google.com/group/v8-users/browse_thread/thread/33a69c51d8023ced

This will make it easier for Linux distributions
to ship with system-provided V8 library.

Patch by Pawel Hajdan, Jr. <phajdan.jr@chromium.org>

Review URL: http://codereview.chromium.org/6092006


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6153 ce2b1a6d-e550-0410-aec6-3dcde31c8c00