On OpenBSD the compiler complains that calling basename(3) would lose
const qualifier. basename(3) is defined as
char *basename(char *);
and can, accorgindly to the POSIX.1, modify the passed string.
This uses the .get_file() method. The check is necessary because
file_name could be a directory, in which case .get_file() would return
an empty string. The .get_base_dir().get_file() idiom is already used.
The usage of get_file() and the check were suggested by theraot, thanks!