Only attempt to find Qt4 if there is at least one qt device enabled.
Only attempt to find Qt4 if there is at least one qt device enabled.

svn path=/trunk/; revision=10091

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot