Since on Octave 2.1.53 glob returns a cell array instead of a padded char array, call char() on the result of glob(), such that toggle_plplot_use will work on both Octave 2.1.50 and 2.1.53.
Since on Octave 2.1.53 glob returns a cell array instead of a padded char
array, call char() on the result of glob(), such that toggle_plplot_use will
work on both Octave 2.1.50 and 2.1.53.

Thanks to Brian D. Wright <bdwright@phy.ucsf.edu> for the fix.

svn path=/trunk/; revision=5243

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