Replace C calls by C++ calls to remove direct dependence on libplplot that was screwing up make check.
Replace C calls by C++ calls to remove direct dependence on libplplot that
was screwing up make check.

svn path=/trunk/; revision=5660

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