Linking in clang on Windows

We haven't found any major language ABI incompatibilities between 2010,
2012, and 2013 yet, so we haven't had to add any version specific logic.
If we discover an incompatibility and deem it important enough to
implement both versions, then we'll probably use -fmsc-version to decide
which behavior the user gets.

Since clang sets _MSC_VER to be the same as VC++ 11 I will test out with that under Boost if I can.