Minor fixes in dcmdata.

Fixed warnings in dictionary generator instead of in the generated code.
Re-enabled variadic macros for mingw. The option "-Wno-variadic-macros"
should be used instead of disabling this feature.
Fixed some issues with wide character support.

In combineDirAndFilename(), there was an issue with empty directory names
being converted to "." if parameter "allowEmptyDirName" = OFFalse. And in
appendFilenameExtension(), support for 8-bit character file extensions was
missing, e.g. append ".bak" to a wide or 8-bit character string.
Added test for OFStandard::appendFilenameExtension().
Fixed issue with untyped NULL reported by MSVC++.