Actually export functions when building DLL on Windows.
Variadic macros are only available in MSVC 2008 and higher.
Support threading API when compiling on Windows using MSVC.
Also export "show..." functions (to be used by dec265 sample).
Add "LIBDE265_API" to functions in .c file, only export when compiling using MSVC for now.
Added Makefiles for Visual Studio (tested with 2003, probably also works with other versions, too).