Check for actual qhull header just in case the platform has the qhull library installed but not the headers for that library.
Check for actual qhull header just in case the platform has the qhull library
installed but not the headers for that library.

svn path=/trunk/; revision=6887

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