This fixes a problem with how CMake reports a missing libwps.  I think this is a bug I introduced myself when upgrading things to the new wpg libraries.  Essentially, instead of reporting that libwps was missing, it was reporting that the libwpg stream library was missing.  While it's true that libwps IS the libwpg stream library, most distributions package it as libwps.  

Additionally, the cmake module was not correctly checking the cache to see whether libwps had been installed.  So if someone ran cmake, got the error, and then installed wps, the error would come up even if the problem had been corrected. 

This patch fixes both glitches.

KOffice builds and runs both with and without libwps and CMake correctly detects whether libwps is present.