Claim compatibility with VS 2012 by default

This will define _MSC_VER to 1700 by default and avoid linker errors
from /failifmismatch linker directives in the C++ standard headers.

Most people trying out the Visual Studio integration are using 2012,
since that's the only version that clang-format works with.  This way
they don't have to pass funky -Xclang -fmsc-version=1700 flags just to
link against the standard C++ runtime.