[Interest] moc: undefined interface - only on macOS?

[Matthew Woehlke]

I am trying to build https://github.com/kitware/qtextensions. It builds
fine on Windows and Linux, but on macOS, I get strange errors (from moc)
when trying to build the designer extensions:

Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtOverlayWidgetInterface.h:18:
Error: Undefined interface
/Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtDoubleSliderInterface.h:18:
Error: Undefined interface
/Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtProgressWidgetInterface.h:18:
Error: Undefined interface
/Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtThrobberInterface.h:18:
Error: Undefined interface
/Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtExpanderInterface.h:18:
Error: Undefined interface
/Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtColorButtonInterface.h:18:
Error: Undefined interface
/Users/kitware/Jenkins/workspace/FletchMacPR/build/buildqtExtensions/designer/qtSqueezedLabelInterface.h:18:
Error: Undefined interface

Anyone have any idea why this would not work on macOS, and *only* on macOS?

The lines that are producing the errors are all:

    Q_INTERFACES(QDesignerCustomWidgetInterface)

The classes all derive (indirectly and through a templated class) from
QDesignerCustomWidgetInterface. I'm using Qt 5.11.2 built from sources.

-- 
Matthew