Resolve "Man pages"

Closes #36 (closed)

inc/lektor/cmd.h

@@ -21,3 +21,7 @@ struct lkt_cmd_opt {
 /* Parse the command line with the list of options. No parameter may be NULL.
    Does not return. */
 noreturn void lkt_cmd_parse(struct lkt_cmd_opt *opts, int argc, const char **argv);
+
+/* Must be setted for the lkt_cmd_parse function to display the correct help
+   in case of errors. */
+extern const char *executable_name;