For issue #4371 ("Subversion requires Serf 1.2.1"), teach the build
system to expect Serf 1.2.1 now.

* INSTALL,
* configure.ac,
* get-deps.sh,
* build/generator/gen_win.py
  Bump Serf required version to 1.2.1.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489114 13f79535-47bb-0310-9956-ffa450edef68