Allow building Subversion 1.8.x against Serf 1.3.0 and higher by allowing
compilation against an already compiled zlib and serf for this specific
scenario.

Compiling serf requires an already compiled zlib, so we can't just (re)compile
zlib after serf.

On trunk this is handled via the improved library detection code.

* build/generator/gen_win.py
  Drop 'serf' and 'zlib' targets for serf >= 1.3.0
  Link against libeay32 and ssleay32 for serf >= 1.3.0
  Fix zlib library name when using an already compiled zlib.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x-serf-1.3+-windows@1517123 13f79535-47bb-0310-9956-ffa450edef68