On OpenBSD, param.h needs to be included before sysctl.h (http://www.openbsd.org/cgi-bin/man.cgi?query=sysctl&sektion=3).
I did it the following way (for trunk@5187):

----------------------8<----------------------
diff -Nur trunk/configure.in.orig trunk/configure.in
— trunk/configure.in.orig 2007-12-18 09:34:19.000000000 +0100
+++ trunk/configure.in 2007-12-18 10:37:51.000000000 +0100
@@ -275,6 +275,11 @@
[
#include <stdlib.h>
#include <sys/types.h>
+
+#ifdef HAVE_SYS_PARAM_H
+#include <sys/param.h>
+#endif /* HAVE_SYS_PARAM_H */
+
#include <sys/sysctl.h>
#include <vm/vm_param.h>
#include <sys/vmmeter.h>
@@ -371,6 +376,11 @@
AC_TRY_COMPILE(
[
#include <sys/types.h>
+
+ #ifdef HAVE_SYS_PARAM_H
+ #include <sys/param.h>
+ #endif /* HAVE_SYS_PARAM_H */
+
#include <sys/sysctl.h>
#include <unistd.h>
#include <time.h>
@@ -395,6 +405,11 @@
AC_TRY_COMPILE(
[
#include <sys/types.h>
+
+ #ifdef HAVE_SYS_PARAM_H
+ #include <sys/param.h>
+ #endif /* HAVE_SYS_PARAM_H */
+
#include <sys/sysctl.h>
],
[
@@ -416,6 +431,11 @@
AC_TRY_COMPILE(
[
#include <sys/types.h>
+
+ #ifdef HAVE_SYS_PARAM_H
+ #include <sys/param.h>
+ #endif /* HAVE_SYS_PARAM_H */
+
#include <sys/sysctl.h>
],
[
@@ -436,6 +456,11 @@
AC_TRY_COMPILE(
[
#include <sys/types.h>
+
+ #ifdef HAVE_SYS_PARAM_H
+ #include <sys/param.h>
+ #endif /* HAVE_SYS_PARAM_H */
+
#include <sys/sysctl.h>
],
[
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Greetings
Sebastian