Remove "plplot/" prefix from plplot include file names.  That style is no longer correct and leads to trouble for builds where nothing is installed yet.
Remove "plplot/" prefix from plplot include file names.  That style is
no longer correct and leads to trouble for builds where nothing is installed
yet.

svn path=/trunk/; revision=6014

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