When using CMake GUI, the process is:
* Select source code directory
* Select build directory (I use a 'build' subdirectory of the source code)
* Press Configure to get a list of default settings
* Evaluate options displayed, make any changes required
* IMPORTANT: If you change any options, you need to press Configure again
* Press Generate

Multiple times I have forgotten to press Configure again before pressing Generate, leading to files not compiling as expected.