+ # Guess a possible nit_dir.
+ #
+ # It uses, in order:
+ #
+ # * the option `opt_nit_dir`
+ # * the environment variable `NIT_DIR`
+ # * the runpath of the program from argv[0]
+ # * the runpath of the process from /proc
+ # * the search in PATH
+ #
+ # If there is errors (e.g. the indicated path is invalid) or if no
+ # path is found, then an error is displayed and the program exits.
+ #
+ # The result is returned without being assigned to `nit_dir`.
+ # This function is automatically called by `process_options`
+ fun locate_nit_dir: String