Fix definition of V8_LIBRT_NOT_AVAILABLE in v8.gyp since we test for it using #elif in time.cc.

TBR=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/23718004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16460 ce2b1a6d-e550-0410-aec6-3dcde31c8c00