warning: multi-character character constant

That's a somewhat obscure feature which has been in C since long before ANSI/ISO; 'xxxx' is not a null-terminated character string like "xxxx" which would be stored byte-wise.  It depends on your system being capable of storing that group of characters in a way similar to that suggested by your quoted usage, so it's non-portable.  Of course, nowadays, 4 characters are expected to fit in a long, but there's also the possibility that a string was meant rather than a multi-character constant.

编译以下代码时出现的问题。

#include <getopt.h>
#include <stdio.h>
#include <stdlib.h>
#include "args.h"

int main (int argc, char * argv[])
{
        int next_opt;
        const char * const short_opts = "hl:";
        const struct option long_opts [] = {
                {"help", 0, NULL, 'h'},
                {"load", 1, NULL, 'l'},
                {NULL, 0, NULL, 0}
        };

        prgm_name = argv[0];

        do {
                next_opt = getopt_long(argc, argv, short_opts, long_opts, NULL);
                switch (next_opt) {
                case 'h' :
                        prt_usage(stdout, 0);
                case 'l' :
                        break;
                case '?' :
                        prt_usage(stderr, 1);
                case '-1' :

                 /*不同机器对被单引号引起的多个字符的处理方式会有所不同。故而，编译器会报 

                    出以上警告信息。*/                        
                        break;
                default :
                        abort();
                }
        } while (next_opt != -1);
        return 0;
}

void prt_usage (FILE * stream, int exit_code)
{
        fprintf (stream, "Usage : %s options.\n", prgm_name);
        fprintf (stream, "-h --help Displays this usage information.\n" \
                "-l int dur --load int dur Set interval and duration of average load.\n");
        exit (exit_code);
}