S3TC (sometimes called DXTn or DXTC) is a proprietary image compression algorithm. This was patented (US Patent 5,956,431) and thus Mesa cannot use it directly nor external library can enter Debian archive. If you want to have libtxc-dxtn installed you have to do it yourself.
I have prepared Debian package with libtxc-dxtn. Its also available from debian-multimedia.org repository.
Here comes installation instruction (if you prefer to build package from sources).
$ dget http://www.burghardt.pl/debian/libtxc-dxtn_0.070518-1.dsc
$ dpkg-source -x libtxc-dxtn_0.070518-1.dsc
Go to libtxc-dxtn-0.070518 directory:
$ cd libtxc-dxtn-0.070518
# dpkg -i libtxc-dxtn0_0.070518-1_amd64.deb
Test S3TC support with glxinfo:
$ glxinfo | grep s3tc GL_EXT_texture, GL_EXT_texture3D, GL_EXT_texture_compression_s3tc, GL_SGIS_texture_edge_clamp, GL_SGIS_texture_lod, GL_S3_s3tc
Additional libtxc-dxtn installation guides: