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.
Using Wine introduces new threats to your Linux box. One day every computer virus prepared to run under Windows will smoothly run under Wine. Some time ago I wrote ClamFS, a FUSE-based user-space file system for Linux with on-access anti-virus file scanning. It uses well known Open Source virus scanner – Clam AntiVirus. ClamFS was designed to protect Samba shares and FTP servers with upload enabled. Here is small guide how to configure ClamFS to protect Wine installation form viruses.