BUG #8271: Configure warning: sys/ucred.h: present but cannot be compiled

The following bug has been logged on the website:

Bug reference:      8271
Logged by:          Emre Hasegeli
Email address:      emre@hasegeli.com
PostgreSQL version: 9.1.9
Operating system:   OpenBSD 5.3 amd64
Description:

Configure warning on OpenBSD 5.3:


checking sys/ucred.h usability... no
checking sys/ucred.h presence... yes
configure: WARNING: sys/ucred.h: present but cannot be compiled
configure: WARNING: sys/ucred.h:     check for missing prerequisite
headers?
configure: WARNING: sys/ucred.h: see the Autoconf documentation
configure: WARNING: sys/ucred.h:     section "Present But Cannot Be
Compiled"
configure: WARNING: sys/ucred.h: proceeding with the preprocessor's result
configure: WARNING: sys/ucred.h: in the future, the compiler will take
precedence
configure: WARNING:     ## ---------------------------------------- ##
configure: WARNING:     ## Report this to pgsql-bugs@postgresql.org ##
configure: WARNING:     ## ---------------------------------------- ##
checking for sys/ucred.h... yes


It appears on 9.1.9, 9.2.4, 9.3beta2; does not appear on 9.0.13.