Maybe I'm the only one, but one thing I have done all (three...) times I have compiled Sigil is to read "same level as source" as the same level as the "src" directory, not the unpacked Sigil-x.y.z-directory. No biggie, but maybe it could be clarified a bit in the doc?
In other words....
- I download Sigil-0.9.5.zip
- unzip Sigil-0.9.5.zip
- cd Sigil-0.9.5
- ls -lh
- Open docs/BuildingOnLinux.md
- Read and get to the create "build" same level src.
- As I see a directory "src" in front of me, I create a directory called "build" at the same level
- I "cd build" and do the first cmake, get the error message, get confused for 3 seconds and get it, "cd .." "mkdir build", and continue...
Now when I have written this, I'm sure I will remember to next time.