問題は、リンクまたはインクルードのいずれかの方法です....ライブラリlibircclient.hは見つかりましたが、ライブラリで使用されている関数は見つかりません....
#include <stdio.h>
#include <stdarg.h>
#include <string.h>
#include <stdlib.h>
/*#include "./libircclient-1.6/include/libircclient.h"*/ 
#include "libircclient.h"
typedef struct
{
    char    * channel;
    char    * nick;
} irc_ctx_t;
void addlog (const char * fmt, ...)
{
    FILE * fp;   // File-Zeiger
    char buf[1024];
    va_list va_alist; 
    va_start (va_alist, fmt); 
#if defined (WIN32)
    _vsnprintf (buf, sizeof(buf), fmt, va_alist);  
#else                                                         
    vsnprintf (buf, sizeof(buf), fmt, va_alist); 
#endif
    va_end (va_alist); 
    printf ("%s\n", buf);
    if ( (fp = fopen ("irctest.log", "ab")) != 0 ) // High level, daher File-Zeiger
    {
        fprintf (fp, "%s\n", buf);
        fclose (fp);
    }
}
void dump_event (irc_session_t * session, const char * event, const char * origin, const char ** params, unsigned int count)
{
    char buf[512];
    int cnt;
    buf[0] = '\0';
    for ( cnt = 0; cnt < count; cnt++ )
    {
        if ( cnt )
            strcat (buf, "|");
        strcat (buf, params[cnt]);
    }
    addlog ("Event \"%s\", origin: \"%s\", params: %d [%s]", event, origin ? origin : "NULL", cnt, buf);
}
void event_join (irc_session_t * session, const char * event, const char * origin, const char ** params, unsigned int count)
{
    dump_event (session, event, origin, params, count);
    irc_cmd_user_mode (session, "+i");
    irc_cmd_msg (session, params[0], "Hi all");
}
void event_connect (irc_session_t * session, const char * event, const char * origin, const char ** params, unsigned int count)
{
    irc_ctx_t * ctx = (irc_ctx_t *) irc_get_ctx (session);
    dump_event (session, event, origin, params, count);
    irc_cmd_join (session, ctx->channel, 0);
}
void event_privmsg (irc_session_t * session, const char * event, const char * origin, const char ** params, unsigned int count)
{
    dump_event (session, event, origin, params, count);
    printf ("'%s' said me (%s): %s\n", 
        origin ? origin : "someone",
        params[0], params[1] );
}
....
gcc irctest.c
/tmp/ccbleNaD.o: 関数 `event_join' 内: irctest.c:(.text+0x2b9):
「irc_cmd_user_mode」への未定義の参照 irctest.c:(.text+0x2d4):
`irc_cmd_msg' への未定義の参照 /tmp/ccbleNaD.o: 関数内
`event_connect': irctest.c:(.text+0x2fe): への未定義の参照
`irc_get_ctx' irctest.c:(.text+0x33b): 未定義参照
irc_cmd_join' /tmp/ccbleNaD.o: In functiondcc_recv_callback':irctest.c:(.text+0x43c): 「irc_dcc_msg」への未定義の参照
irctest.c:(.text+0x49b): `irc_dcc_msg' への未定義の参照
irctest.c:(.text+0x4a7): `irc_strerror' への未定義参照 ....
collect2: ld gab 1 als Ende-Status zurück