A guy at IRC just tried to install compiz and found that problem:
root:# slapt-get -i compiz
Reading Package Lists... Done
Excluding compiz, use --ignore-dep to override
The following packages have unmet dependencies:
compiz: Depends: mesa >= 7.2-i586-1vl60
The following packages have been EXCLUDED:
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
I searched in my box, and IO have it installed, and when I tried
slapt-get --search mesa
Package Mirror: http://vectorlinux.osuosl.org/veclinux-6.0/packages/
Package Priority: Default
Package Location: ./x
but looking in http://vectorlinux.osuosl.org/veclinux-6.0/packages/x/
there's mesa 7.0.2 (for i486, so the slackware version), but not the 7.2 version.
He managed to install compiz-fusion , but anyway, something strange is happening with mesa, isn't it?