I already posted a solution about compiling the Steam plugin for Pidgin on a 64Bit Linux.

Recently, the whole plugin has been upgraded to version 1.1 which fixes some problems with duplicate messages and authentication errors.  The instructions remain the same.

I added a new .so library for this version. You can download it here: CLICK.

If you are running Slackware you can build a package for your system with this Slackbuild: http://schmatzler.de/my_slackbuilds/pidgin-opensteamworks/