Thanks @lassoan 
!
I’m not sure where I am supposed to find the build script in the second link you sent. But the links still helped. I can get Slicer to compile now. I ran the following:
cd S4D
export LD_LIBRARY_PATH=/media/vaughan/workspace/lnx/devel/Support/qt-everywhere-build-5.10.0/lib
export PKG_CONFIG_PATH=/media/vaughan/workspace/lnx/devel/Support/qt-everywhere-build-5.10.0/lib/pkgconfig
export PATH=/media/vaughan/workspace/lnx/devel/Support/qt-everywhere-build-5.10.0/bin:$PATH
cmake -DCMAKE_BUILD_TYPE:STRING=Debug -DQt5_DIR=/media/vaughan/workspace/lnx/devel/Support/qt-everywhere-build-5.10.0/lib/cmake/Qt5 …/S4
And the rest of the build was ok. 
Edit: Though inconvenience that I have to export the LD_LIBRARY_PATH to run Slicer from Terminal.
Normally setting the Qt5_DIR in CMake should be enough though… no?
Edit again: I’m going to try setting the path variable similar to here.