msvc: skip copying Data dir and precompiled dlls to Binary dir if they are already there (unless the source files are newer).

Since the move to VS2013, the Data directory was unconditionally copied to the Binary directory each time the DolphinWX project was built. Previously, `xcopy` was used, and only copied files if the version in the Data/ was newer than the one in the Binary/. This restores that behavior. Practically, this means that gameini files aren't overwritten if you have local changes.