Fix build of OCaml documentation.  Note, similar changes need to be made to ocaml/plcairo/CMakeLists.txt which I leave to Hez.
Fix build of OCaml documentation.  Note, similar changes need to be
made to ocaml/plcairo/CMakeLists.txt which I leave to Hez.

svn path=/trunk/; revision=10527

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