Restrict use of $(sources) name to actual source code.  Rationalize the way that distributed data files are specified.
Restrict use of $(sources) name to actual source code.  Rationalize the
way that distributed data files are specified.

svn path=/trunk/; revision=6130

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