ENH: cmake: Add FindVcvars CMake module

This module allows to look up path of 'vcvars(32|64|amd64).bat' scripts.

It will facilitate the building of Slicer extension including python
modules having compilation steps.

It is being contributed to upstream CMake as
https://gitlab.kitware.com/cmake/cmake/merge_requests/899

git-svn-id: http://svn.slicer.org/Slicer4/trunk@26057 3bd1e089-480b-0410-8dfb-8563597acbee