cmake: link release builds statically against run-time library on windows (use /MT instead of /MD)

This should fix the problem of snapshot builds created with MSVC 2012 not
working for users with MSVC 2010, etc.

Differential Revision: http://llvm-reviews.chandlerc.com/D2157

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194589 91177308-0d34-0410-b5e6-96231b3b80d8