Building libxcb-1.1 on Solaris2.6 also fails because on this
old system, the newer getaddrinfo() / freeaddrinfo() are not
available and AF_INET6 is not defined. (You may ask yourself
why to bother with such an old system. Well, among others, some
old hardware drivers are not available for newer OS versions).
A solution seems simple enough to be worth considering: On systems
which don't have getaddrinfo(), use the older gethostbyname()
and exclude the AF_INET6 containing blocks (since not available
on these systems, anyway). Maybe '#ifdef AF_INET6' could be used
for both conditional changes without any change in 'configure'
or else configure could work out if getaddrinfo() is available
(#define HAS_GETADDRINFO)...
When I applied the following changes to xcb_auth.c and xcb_util.c
then compilation worked, but maybe someone with more experience
in network programming should check and test:

xcb_auth.c, line 95, switch construct in function get_authptr():
conditionally exclude the whole 'case AF_INET6:' block (10 lines)
with an #ifdef AF_INET6 / #endif pair.

xcb_util.c, line 201 , function _xcb_open_tcp():
conditionally replace the whole function, using some construct
like #ifdef HAS_GETADDRINFO (or maybe just #ifdef AF_INET6 ?)
/ #else / #endif, with some alternative function similar to:
...
#else
/* //### for older systems, for example Solaris2.6,
   //### rewrite to use gethostbyname() instead of getaddrinfo() which
   //### was not yet available on these old systems.
*/
static int _xcb_open_tcp(char *host, char *protocol, const unsigned short port)
{
    int fd = -1;
    struct hostent *hostent_results;
    struct sockaddr sock_addr;
    char **pAddr; 
    char *bracket;

    if (protocol && strcmp("tcp",protocol))
        return -1;

    /* DO NOT allow IPv6 addresses enclosed in brackets. */
    if(host[0] == '[' && (bracket = strrchr(host, ']')) && bracket[1] == '\0')
        return -1;

    if ( NULL == (hostent_results = gethostbyname(host)) )
        return -1;

    for(pAddr = hostent_results->h_addr_list; *pAddr; pAddr++)
    {
        fd = socket(AF_INET, SOCK_STREAM, 0);
        sock_addr.sa_family = AF_INET;
        memcpy(sock_addr.sa_data, *pAddr, hostent_results->h_length); 
        if(fd >= 0 && connect(fd, &sock_addr, hostent_results->h_length) >= 0)
            break;
        fd = -1;
    }
    return fd;
}
#endif

[compiles / allows libxcb to be built - but function is not tested!]