gWidgetstcltk needs tcl/tk 8.5

The gWidgetstcltk package now needs tcl/tk version 8.5 or newer
installed. This change was made when R 2.7.0 shipped this version for
windows. The 8.5 version allows for many more widgets, that prior to
this were hacked or ignored in gWidgetstcltk.

For windows this is not issue, for linux users not a big issue, for
Mac OS X users, one needs to (I think) compile the tcl/tk libraries
from source (www.tcl.tk). This has been confirmed to work for
gWidgetstcltk under OS X 10.5, but YMMV.