Thanks for the hint. I checked some things to make sure that everything is set to x64, but then I noticed that the emscripten target is set to x86! Some more googling indicated that there appears to be no 64 bit compiler for emscripten (web assembly). That means I’ll have to rebuild everything for x86… 
Now I’m wondering how to set the build target to x86: cmake-gui does not show any build target, so I assume the default is x64. I guess I’ll find out eventually, but if someone reads this within the next half hour I would appreciate a tip.