Added quotes in case the variable is empty.

CTKPluginUseFile.cmake.in

@@ -1,5 +1,5 @@
 # List all plugins
-SET(CTK_PLUGIN_LIBRARIES @CTK_PLUGIN_LIBRARIES@)
+SET(CTK_PLUGIN_LIBRARIES "@CTK_PLUGIN_LIBRARIES@")
 
 # Plugin specific include directories
 @CTK_PLUGIN_INCLUDE_DIRS_CONFIG@