If False, then the C mode associated with the SmartEiffel level
will be selected; otherwise, the user has explicitely used the
-c_mode option; this one will be used.
The substitution is performed in line.
When given the path information is used to emit an error message when the variable does not exists.
If any, substitute in line some "system.se" well-known key (i.e. "sys", "bin" or "short").
If case of unknow key, uses echo.getenv instead.
If False, then the C mode associated with the SmartEiffel level
will be selected; otherwise, the user has explicitely used the
-c_mode option; this one will be used.