In the Windows build: make it much easier to apply a define through
all projects when building via project systems such as Visual Studio
by adding an optional -D option to gen-make.py.

* build/generator/gen_win.py
  (get_win_defines): Add cpp_defines when set.
  
* build/generator/gen_win_dependencies.py
  Initialize cpp_defines. Parse -D options. 
  
* gen-make.py
  Allow and document -D option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567046 13f79535-47bb-0310-9956-ffa450edef68