Define gdImagePalettePixel macro if not already defined, so that people with stable version of gd can compile this.
Define gdImagePalettePixel macro if not already defined, so that people
with stable version of gd can compile this.

svn path=/trunk/; revision=3919

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