Welcome to Linux Questions.
Well, you get all these errors, when 'cmake'
cannot find anything.
You must have a "build directory" outside
the top - directory of the application.
a) you have "/home/xert/lancelot" and
b) a build : /home/xert/build-lancelot
To make the stuff : 1) 'cd build-lancelot'
2) 'cmake ../lancelot/' => pay attention <= EDIT !
to the 2 dots, to get the path to "lancelot"
3) 'make' ... 'su' ... 'make install'
As non of your questions are Debian specific,
I will suggest starting a new thread with
the mouse issue in "Linux > Hardware"
Good Luck !
P.S.: There is a package for lancelot :