A release of the DIRT interface builder by Richard Hesketh works with X11R5 and includes some support for the Motif widget set. It generates WCL-1.1 code.
DIRT is known to be outdated.
It is available through comp.sources.x archives. Semantics can be coded in C or in Ness, the AUIS extension language. Unix World and Unix Review often carry articles. OI is built directly on top of Xlib and is unencumbered by runtime royalties to any party. ObjectBuilder and OI are very extensible, actively facilitating the use of subclassing to create new, reusable, user interface components.