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.
ZSNES is a GPL licensed emulator of the Super Famicom and Super Nintendo Entertainment System (SNES) video game systems. Large part of ZSNES is written in assembly. Fortunately it works under GNU/Linux. Debian has a zsnes package, but unfortunately only for i386. Binary package is not available for AMD64. So? Lets build one!