In older versions of tidy, these headers were defined. Now, they are
renamed to tidybuffio.h and tidyplatform.h. This may be more of a
consistent naming scheme, but it breaks current software.
Re-add these headers and add compile time warning when such a header
is used.
Not sure how this has lasted so long, but I guess there are not too many
users wanting to specifically use the static tidy library.
Now it is possible to define TIDY_STATIC and the TIDY_EXPORT macro will
just be 'external', suitable for linking with the static library, instead
of adding the _imp_ MSVC __declspec( dllimport )...
This fix does not effect other than the WIN32 platform build.
