mingw32-nsiswrapper with gtk3

[Michael Cronenworth]

Erik van Pienbroek wrote:
> I think a better solution would be
> to change the script so that it supports both '--with-gtk2' and
> '--with-gtk3' so that users can decide for themselves whether they want
> to bundle gtk2 or gtk3 with their installers.

+1

Is GTK3 even ready for Win32/64? Everything I've seen says no even 
though I see we have MinGW packages of it in the repos.