The added cmake-parameter is: -DLYX_EXTERNAL_MYTHES=<value> where <value> is one of AUTO (Default) Search first on system for mythes (lib and include) ON Use installed only (errors if not installed) OFF Compile the provided source in 3rdparty
For some reason, I mistyped "myspell" instead of "mythes" as a reflex. I thought that I fixed all occurences, but it appears that I missed one...
Move it to 3rdparty/ directory alongside the other ones.