Summary:
MSVC allows co-installing multiple version of the compiler and
vcvarsall.bat will use the latest one by default. This patch adds
a new configuration option that allows user to choose an older toolset
to use.

This is useful when trying to reproduce a build failure against an older
version of MSVC.

Test Plan: Tested locally to force 14.14.26428 toolset while having 14.16.something installed.

Reviewers: vonreth

Reviewed By: vonreth

Differential Revision: https://phabricator.kde.org/D17636