In Cygwin projects using libtool, we always have to add -no-undefined
iLDFLAGS.  This is some old safe-guard in libtool to remind developers
that when creating Windows DLLs, all external symbols must be resolved.

Fortunately, libtool allows this flags also on other platforms which
don't require its usage.

Patch is attached.  It would be nice if that's ok for inclusion.