Properly handle the 'when' option in build.conf on Windows. This filters
a few projects that currently can't be build on Windows.

* build/generator/gen_win.py
  (get_install_targets): Filter projects that are disabled.

* build/generator/gen_win_dependencies.py
  (_windows_when): New variable.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1524029 13f79535-47bb-0310-9956-ffa450edef68