Merge topic 'avoid-MSVC-per-version-vars'

9ab966a7 FindRuby: Use `MSVC_VERSION` instead of `MSVC##`
69b9fde1 FindGTK2: Use `MSVC_VERSION` instead of `MSVC##`
3fe31822 FindBoost: Avoid mentioning discouraged `MSVC##`` variable

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !610