Reading through the cmake quick start, you can specify variable on a command line:
cmake -DVARIABLE1=value1 -DVARIABLE2=value2 ...
Otherwise, set command in the cmake script is probably what you want, see the reference manual. To set the environment variable PATH, do:
set(ENV{PATH} "/home/martink")
To set normal variable, do:
set(variable "value")
Not sure which ones you have to set, probably the environment ones.
That said, setting environment variable prior to calling cmake is often the easiest solution to solve the problem. If you want a cross-platform way to do this that doesn’t depend on the syntax of a specific shell to set environment variables, there is the cmake -E env command.