Ok. I have decided to use:

#if defined(sun) && if defined(sparc) && !defined(__svr4)

instead of defined(sunos4).  interfaces/libpq/libpq-fe.h and
include/c.h have been modified(see included patches).

Another porblems I have found are:

o SunOS lacks strtoul(). to fix this I stole strtoul.c from FreeBSD
and place it under backend/port. necessary modifications have been
also made to backend/port/Makefile.in, include/config.h.in and
configure.in (see included patches).