Ensure -DUSE_PERLIO gets into Config's ccflags when using dmake on Windows

The -DUSE_PERLIO flag was being used to build perl itself, but amazingly
didn't end up in Config's ccflags, potentially causing trouble later when
building XS extensions. (The problem isn't new: it's been this way ever
since PerlIO support for Windows was added by e601c439, way back in
November 2000.)