О, на пятый день нашёл ответ! Всем, кто собирался ответить, но не успел, большое спасибо! ;)
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The reason to the build failure is that the ipf headers,
ip_compat.h, ip_fil.h and ip_nat.h are no longer installed on
recent FreeBSD releases.
They used to be install in old times. (I was able to build squid
with transtarent-ipf support on a 4.1-RELEASE machine at that time.)
I figured out this problem when I replaced my old proxy-box with
new machine and completely fresh installed 4.8-RELEASE.
I'm not sure, when the FreeBSD core team has changed their policy
about ipf headers, and suddenly changed "NOT" to install them.
These headers of course exist in the kernel source tree
/usr/src/sys/contrib/ipfilter/
but only used at kernel and kernel modules compile time.
In order to fix, there two solutions.
1. Ask FreeBSD core team (may be kernel maintainer) to re-enable
the installation of ipf headers to proper place (/usr/include).
2. Some how make a simbolyc link to /usr/src/sys/contrib/ipfilter/*.h
at squid port build time.
My workaround is copying ipf headers from kernel source tree to
/usr/include everytime when make a new make world, but this is
very odd. I suppose port system should utilize those headers
by itself.
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