Without -no-undefined, libtool refuses to create the shared library (dll) on Cygwin, because libtool knows that creating a shared library (dll) on Cygwin does require all symbols to be defined.
Unfortunately but traditionally, by default libtool has to assume a library being created will have undefined symbols.
Hence, if the library to be created is designed to have all symbols defined, libtool needs to be informed about this fact using the -no-undefined flag.
This flag does allow libtool to create a shared library even on platforms known to require all symbols to be defined for shared libraries.

D184: 409_libksba-1.3.0-undef-true.patch