Clean build of pgsql 7.4.5 on HPUX B.11.23 on ia64 with 
--enable-thread-safety fails ...   :(

$uname -a
HP-UX ... B.11.23 U ia64 0144848471 unlimited-user license

$cd postgresql-7.4.5
$./configure --enable-thread-safety --without-readline --without-zlib
...
checking pthread.h usability... yes
checking pthread.h presence... yes
checking for pthread.h... yes
configure: error:
Cannot enable threads on your platform.
Please report your platform threading info to the PostgreSQL mailing lists
so it can be added to the next release.  Report all compile flags, link 
flags,
functions, or libraries required for threading support.
See the comment at the top of src/port/thread.c for more information.

$gcc --version                                                    
gcc (GCC) 3.4.2
Copyright (C) 2004 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.