add_library() may add a "lib" prefix to the plugin's filename, which causes it not to be found.
Using kde4_add_plugin() instead again fixes that.
This change to add_library() was (accidentally?) backported from the KF5 version in commit c78183f18201 .
BUG: 386547
FIXED-IN: 4.8.2
Intended for the 4.8 branch only, of course.