On Windows, make it possible to build Subversion against Serf 1.3.0 and
higher by allowing compilation against an already compiled serf.

* build/generator/gen_win_dependencies.py
  (_find_serf): Set is_src to False when version >= 1.3.0.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1517192 13f79535-47bb-0310-9956-ffa450edef68