CMake: make building with /MT an option instead of always forcing it

for release builds.

This is a follow-up to r194589. Aaron pointed out that building
libraries with /MT and using them in an application that uses a
different run-time library can be a bad idea.

Move the option to build with /MT behind a CMake option so it can be
turned on selectively, such as when building the toolchain installer.

llvm-svn: 194596