For Visual C++ _isnan is defined in float.h, therefore we need to include it.
For Visual C++ _isnan is defined in float.h, therefore we need to include it.

svn path=/trunk/; revision=9633

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