bug#7643: Build failure for SGI IRIX MIPS

[Chong Yidong]

Nelson H. F. Beebe reports a build failure of the 23.2.92 pretest on SGI
IRIX MIPS.

The last Emacs version to build ok was 21.3 in January 2004, so this
appears to be a long-standing bug.

> On IRIX, compilation fails in src/process.c like this:
>
>       cc-1367 c89: ERROR File = process.c, Line = 2431
>         A pointer to an incomplete class type is not allowed.
>
>               uint16_t *ip6 = (uint16_t *) &sin6->sin6_addr;
>                                             ^
>
>       cc-1367 c89: ERROR File = process.c, Line = 2432
>         A pointer to an incomplete class type is not allowed.
>
>               len = sizeof (sin6->sin6_addr)/2 + 1;
>                             ^
>
>       cc-1367 c89: ERROR File = process.c, Line = 2435
>         A pointer to an incomplete class type is not allowed.
>
>               p->contents[--len] = make_number (ntohs (sin6->sin6_port));
>                                                                        ^
>
>       cc-1070 c89: ERROR File = process.c, Line = 2489
>         The indicated type is incomplete.
>
>                 return sizeof (struct sockaddr_in6);
>                                ^
>
>       cc-1367 c89: ERROR File = process.c, Line = 2546
>         A pointer to an incomplete class type is not allowed.
>
>                 uint16_t *ip6 = (uint16_t *)&sin6->sin6_addr;
>                                              ^
>
>       cc-1367 c89: ERROR File = process.c, Line = 2547
>         A pointer to an incomplete class type is not allowed.
>
>                 len = sizeof (sin6->sin6_addr) + 1;
>                               ^
>
>       cc-1367 c89: ERROR File = process.c, Line = 2549
>         A pointer to an incomplete class type is not allowed.
>
>                 sin6->sin6_port = htons (i);
>                 ^
>
>       cc-1070 c89: ERROR File = process.c, Line = 4388
>         The indicated type is incomplete.
>
>             struct sockaddr_in6 in6;
>                                 ^
>
>       8 errors detected in the compilation of "process.c".