Change default behaviour of get_deps.sh from downloading gtest
library to making it an optional target.

* get-deps.sh
  (usage): Add gtest to list of possible arguments.
  (get_deps): Remove gtest from list of default downloads.

Patch by: Gabriela Gibson <gabriela.gibson{_AT_}gmail.com>


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428689 13f79535-47bb-0310-9956-ffa450edef68