Use __declspec(dllexport) to export symbols (drop gendef) (gtkmm master)

Hi,

This is the master branch counterpart of MR !25 (merged), except that for this case we cease to use gendef.exe entirely in the master branch, because we already require a glibmm installation that contains all the necessary changes for the changes here to be effective. This means that gendef will no longer be used on gtkmm master from this point on.

With blessings, thank you!