To those saying "ignore it": consider that people from the US aren't really at fault for their broken patent system and can't normally do anything about it.
It's not fair to bitch when something is US-only and "we" can't access it, and then turn around and say "fuck em" when the tables have turned.
Great work from the Xonotic developers, I hope this gets included in Mesa soon!
Actually, the preferred way would not be including this in Mesa as ONLY version, but to continue to have support for the s3tc-enabled libtxc_dxtn.
This can be realized for example by Linux distributions setting this to "Provide:" libtxc_dxtn, and shipping this one by default. Then the user can replace it by the "full" libtxc_dxtn with S3TC support by an apt-get command (on Debian/Ubuntu). Alternatively, an alternatives mechanism can be used (update-alternatives) to even allow both packages to be installed at the same time.
Also, Mesa COULD include this code, or reimplement S2TC on their own (in a probably more optimized and faster fashion). In any case, I would prefer support for full S3TC libtxc_dxtn to remain.
It would automatically work, provided your graphics card supports S3TC decoding. In case it does not, Mesa may implement it using S2TC decoding, but even if it will work, it will be slower. You basically don't want software fallbacks to happen.
Quite likely though, your card DOES support S3TC, and libtxc_dxtn will then work for you without fps loss.
The reason that ignoring it isn't a solution is that the US is -- for better or worse -- still one of the largest drivers and absolutely the largest consumer of software, games, and technology in the world. If some random user in some other part of the world gets access to an encumbered algorithm, good for him, but that does nothing for the larger ecosystem. Game developers will still absolutely refuse to waste time targeting Linux if we know that even 60% of the Linux userbase in our target market would be unable to use that software.
So far as S3TC, you already have the option to use a Mesa plugin for it in parts of the world that don't uphold software patents, so quit your damn whining already.