build: windows: add pywinmake and simplify winmake script

Building libjami should now be doable using:
- init   : git submodule update --init
- install: pip install extras/scripts/pywinmake
- contrib: python extras/scripts/winmake.py
- libjami: python extras/scripts/winmake.py --base-dir compat/msvc

NOTE: breaks libjami output location

TODO: fix plugin build

Change-Id: Ic6b25d00103f0ea68d8ab7954f8345f1648e0381