Add checks, rules and variables to the build system for docwriter.
* Running `make' will warn if Python/PIP/docwriter are not available.
* Running `make refdoc' will generate static documentation site on the
current Python environment.
* Running `make refdoc-venv' will generate static documentation site
using a virtual environment, using the pip package `virtualenv'.
The build target is now `windows' instead of `win32'.
Problem reported by Nickolas George <darknova.clan@gmail.com>.
* builds/modules.mk: Don't use `win32' and `win16' (!) but
`windows'.
* builds/windows/detect.mk, builds/windows/win32-def.mk:
s/win32/windows/.