Hello,
It looks like that Knoppix 6.7.1 is missing the header files in the /usr/include/[bits,gnu,sys] folders.
Copying these folders from Knoppix 6.7.0 fixes it. Gcc/ld may also be missing something in order to
compile 64 bits binaries (the pi benchmark as an example: http://h2np.net/pi/pi_record_e.html)
First the 64 bits libraries need to be installed: "sudo apt-get -y --reinstall install amd64-libs-dev"
and downgrade gcc from 4.6 to 4.4: "pushd /usr/bin; sudo mv gcc gcc.old; sudo ln -s gcc-4.4 gcc; popd".
FYI, trying to use gcc 4.6 (the default) instead of gcc 4.4 (downgrade) provides the following error:

gcc -O2 -pedantic -m64 conftest.c
/usr/bin/ld: skipping incompatible /usr/lib/gcc/i486-linux-gnu/4.6.1/libgcc.a when searching for -lgcc
/usr/bin/ld: cannot find -lgcc
/usr/bin/ld: skipping incompatible /usr/lib/gcc/i486-linux-gnu/4.6.1/libgcc_s.so when searching for -lgcc_s
/usr/bin/ld: cannot find -lgcc_s
collect2: ld returned 1 exit status

What is the best way to fix gcc/ld 4.6 ?
Best Regards,
Gilles