As a result, backend/libpq/pqcomm.c and interfaces/libpq/fe-connect.c
fail to compile.

The <netinet/tcp.h> header needs to be preceded by <netinet/in.h>, at
least on IRIX, Solaris and AIX.  The simple configure test fails.
(That header on Linux is idempotent.)

The basic problem is that <netinet/tcp.h> is a BSD header.  The
correct header for TCP internals such as TCP_NODELAY on a UNIX system
is <xti.h>.  By UNIX I mean UNIX95 (aka XPG4v2 or SUSv1) or later.
The current UNIX standard (UNIX98 aka SUSv2) is available online at
<http://www.opengroup.org/onlinepubs/7908799/>.

The fix is to add header support for <xti.h> into configure.in and
config.h.in.

The 2 files which conditionally include <netinet/tcp.h> need also to
conditionally include <xti.h>.

Pete Forman