Fixed https://github.com/msgpack/msgpack-c/issues/147
On VC9 (Visual Studio 2008) and VC10 (Visual Studio 2010), .c files are compiled as C++. Otherwise the files are compiled as C.
Merge pull request #151 from redboltz/fix_build_vc2010

Fixed https://github.com/msgpack/msgpack-c/issues/147