Basically, the Slicer_USE_PYTHONQT cmake variable needs to be passed on to external projects (I verified the bug for EMSegment and MultiVolumeExplorer).

if(Slicer3_USE_PYTHONQT)
add_subdirectory(Py)
endif(Slicer3_USE_PYTHONQT)

Since EMSegment required python, I would suggest we simply disable it if python is not enabled in Slicer.

Topic pushed to https://github.com/sankhesh/Slicer/tree/2458-building-without-pythonqt

An issue should be considered resolved after it has been integrated into the code base. Before doing so, consider the comment I added on github.

jcfr:
Shouldn't it be

Slicer_BUILD_BRAINSTOOLS AND Slicer_BUILD_QTLOADABLEMODULES AND
Slicer_USE_PYTHONQT

instead of

Slicer_BUILD_BRAINSTOOLS AND Slicer_BUILD_QTLOADABLEMODULES;Slicer_USE_PYTHONQT

Fixed as per jcfr's suggestion and pushed topic to:
https://github.com/sankhesh/Slicer/commits/2458-building-without-pythonqt

Thanks for pushing the fix.

Next time would be great to squash the commit before integrating to Slicer. It seems strange to see to commit having the exact same message but different changeset. See r20959 and r20960