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(at)hasegeli(dot)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(at)postgresql(dot)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.