First of all, linux-kernel-headers has been replaced by linux-libc-dev.
Ok. But linux-kernel-headers is still a virtual package. Isn't it?
But either way, the answer is the same: those packages install them in
/usr/include, which is unpacked and accessible to any program being
compiled. linux-source-2.6.* (AFAIK) is not unpacked by default, and
even when it is unpacked, the headers are not accessible under
linux-source-2.6.* is not unpacked by default (at least on my system).
If it was the case, a symlink from /usr/src/linux-source-2.6.*/include
to /usr/include/linux wouldn't do the trick? Mmmhhh... maybe
/usr/include/linux must point to the headers which were used to compile
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