If it wants the version 2.3.6.ds1-13 of libc6 to work, like it is specified in the message shown by you:
"libc6-dev: Depends: libc6 (= 2.3.6.ds1-13) but 2.3.6.ds1-13etch4 is to be installed", you shoul try the following:
It's likely to work if you force the installation of this version.
First of all, download the package here:
http://packages.debian.org/etch/libc6
Then, in the folder where the downloaded packade is located, you type:
# dpkg --force "the_package's_name"
Finally, you can try install libc6-dev again:
# apt-get install libc6-dev