Porteus

hi install the usm package,remeber that porteus and slackware share the same packages roots so this is the try with usm:
bash-4.2# usm -g mesa

The following items were found.
Choose an number to confirm.
ctrl+c to quit

1) mesa-9.1.7-x86_64-1.txz

as beny pointed - i would neve use 3rd part core library in Porteus.
our Mesa is not that old and if you want never then please compile from sources.