Support Visual Studio 2013 as '2013' in the Windows build.
(It was already possible to use the option '12' to create a working
 but sub-optimal solution)

* gen-make.py
  Document VS 2013 value.
  
* build/generator/gen_win_dependencies.py
  (parse_options): Set internal versions for VS 2013.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1533907 13f79535-47bb-0310-9956-ffa450edef68