Make MS Compilers Toolset private

[JimBobSquarePants]

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Description

Fixes #1166

[codecov]

Codecov Report

Merging #1171 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1171   +/-   ##
=======================================
  Coverage   82.39%   82.39%           
=======================================
  Files         684      684           
  Lines       29516    29516           
  Branches     3327     3327           
=======================================
  Hits        24319    24319           
  Misses       4508     4508           
  Partials      689      689           

Flag	Coverage Δ
#unittests	82.39% <ø> (ø)

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