cygwin: add -no-undefined on our side to fix issues with mac OS
So cygwin requires -no-undefined and it was supposed to be harmless on
platforms other than windows, but mac OS doesn't like it.
Instead of adding platform checks to every plugin, i'm going to revert
the changes that added -no-undefined to them, and just do what I should
have done since the beginning: use the cygwin-specific parts of the
pkg-config file to add -no-undefined right next to -lbitlbee
