check sys/sysctl.h seperately, as it requires other headers on at least OpenBSD

configure.in

@@ -154,7 +154,15 @@ dnl Checks for header files.
 AC_HEADER_DIRENT
 AC_HEADER_STDC
 AC_CHECK_HEADERS(fcntl.h sys/ioctl.h sys/time.h unistd.h sys/io.h errno.h)
-AC_CHECK_HEADERS(limits.h kvm.h sys/param.h sys/sysctl.h sys/dkstat.h)
+AC_CHECK_HEADERS(limits.h kvm.h sys/param.h sys/dkstat.h)
+
+dnl check sys/sysctl.h seperately, as it requires other headers on at least OpenBSD
+
+AC_CHECK_HEADERS([sys/sysctl.h], [], [],
+[[#if HAVE_SYS_PARAM_H
+# include <sys/param.h>
+#endif
+]])
 
 dnl Check for particular preprocessor macros