Remove duplicates in the X11_INCLUDE_DIR list since those screw up certain of our modules and also slow down compilation if any of those duplicates leak through to the compile -I options.
Remove duplicates in the X11_INCLUDE_DIR list since those screw up certain
of our modules and also slow down compilation if any of those duplicates
leak through to the compile -I options.

N.B. this logic should be changed to a macro if we ever want to remove
duplicates from other lists.

svn path=/trunk/; revision=7775

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