Basic interface with tcl/tk

1) You should looks to the definition of TCLLIBS in src/Makefile before
   building.

2) Tested only with tcl/tk 8.xx.

3) To test the installation, try 'library(tcltk); example(tcltk)' from a
   running R process. The tcl/tk source of 'density' example (it is in
   exec/density.tcl) has been preparared 'visually' in less than 5 minutes
   using Visual Tcl (http://www.neuron.com/stewart/vtcl).