What is a good C GUI toolkit with an open source form designer?

Currently I only know 2 toolkits that fill these requirements: GTK[1] and XForms[2]. Glade (a GTK form a designer) development has been stagnant in recent years and XForms form designer has been stagnant latest release was in 2014. Anyone any other C GUI toolkit with an opensource form designer available?

[1] https://gtk.org/

[2] http://xforms-toolkit.org/