I opted for Qt 5.4.2 since that's what the Sigil folks use. My eyesight is bad, so at first I was saddened to not see it on the basic "all downloads" page, but those are only "all" of the versions of 5.6.0. There is another link to the archive of previous versions I finally spotted on my third try. Here's the location of the 64-bit Linux installer:
http://download.qt.io/archive/qt/5.4...-x64-5.4.2.run
Once I had that and updated cmake it seems to have compiled fine.