autoconf259 question

[artifex maximus]

Hello!

I try to make a working configure.in but I have the following problem.

I write this:
AC_CHECK_HEADERS([netinet/in_systm.h])
AC_CHECK_HEADERS([netinet/in.h])
AC_CHECK_HEADERS([netinet/ip.h],,, [code to insert])

I run autoheader259 to make config.h.in. Thereafter I run configure
that doesn't found netinet/ip.h and doesn't define HAVE_NETINET_IP_H.

checking netinet/in_systm.h usability... yes
checking netinet/in_systm.h presence... yes
checking for netinet/in_systm.h... yes
checking netinet/in.h usability... yes
checking netinet/in.h presence... yes
checking for netinet/in.h... yes
checking for netinet/ip.h... no

File is exist:
# ls -l /usr/include/netinet/ip.h
-r--r--r--  1 root  wheel  6790 Mar 24 08:50 /usr/include/netinet/ip.h

My system:
FreeBSD * 5.4-STABLE FreeBSD 5.4-STABLE #0: Thu Apr  7 00:27:04 CEST
2005     root@*  i386

What is the problem? Any idea?

Thanks,
artifex