Following up on r1572253, bump serf requirement on Windows.

* build/generator/gen_msvc_dsp.py
* build/generator/gen_vcnet_vcproj.py
  Don't generate serf and zlib projects.

* build/generator/gen_win.py
  Don't create build_zlib.bat
  Stop filtering serf and zlib projects in specific cases.
  
* build/generator/gen_win_dependencies.py
  Remove special is_src support that was only needed for
  this ugly zlib/serf dependency case.
  Bump minimal required serf version.
  
* build/generator/templates/build_zlib.ezt
* build/generator/templates/serf.dsp.ezt
* build/generator/templates/serf.vcproj.ezt
* build/generator/templates/serf.vcxproj.ezt
* build/generator/templates/zlib.dsp.ezt
* build/generator/templates/zlib.vcproj.ezt
* build/generator/templates/zlib.vcxproj.ezt
  Remove unused templates.
  
* build.conf
  Make serf and zlib normal external projects.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572265 13f79535-47bb-0310-9956-ffa450edef68