My first guess is that IAT is assuming GNU-style getopt.h which is very, very different from IRIX's own getopt.h. A quick look (so I'm really only speculating about everything) at the IAC source shows an unconditional include of getopt.h in src/cmdline.c. There are no checks in the configure script to ensure that getopt.h is indeed GNU getopt.h and/or has the necessary features.
And there are, of course, corresponding differences in the implementation of getopt functions in GNU libc vs. IRIX libc. So just copying over the GNU header will not suffice. You'll need to drag in the implementation of some functions, by the looks of it. Perhaps from Gnulib.
So it looks like you've got a fair bit of porting work before it'll compile on IRIX. And that's just for the getopt stuff. Who knows what else the compiler will uncover after you fix that!
And there are, of course, corresponding differences in the implementation of getopt functions in GNU libc vs. IRIX libc. So just copying over the GNU header will not suffice. You'll need to drag in the implementation of some functions, by the looks of it. Perhaps from Gnulib.
So it looks like you've got a fair bit of porting work before it'll compile on IRIX. And that's just for the getopt stuff. Who knows what else the compiler will uncover after you fix that!