Add Tk device driver to our CMake build system, but note tkwin is still disabled.
Add Tk device driver to our CMake build system, but note tkwin is still
disabled.

svn path=/trunk/; revision=6960

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