This plugin is required by many of the Squirrelmail plugins.
Untar in the plugins/ directory
tar -xvzf compatibility-x.x.x.tar.gz
A patch must also be applied.
There are different patches for each version of Squirrelmail, identify your version and apply the appropriate patch.
You can find the version of Squirrelmail on the login page.
patch -p0 < patches/compatibility_patch-1.4.x.diff