Oops. The newest 0.42.5 (autobuilt right on this machine) stops starting up with an empty square window titled "Pd".
The console reads:

$ pd-extended
tk scaling is 1.0419596812001874
tcl: /usr/lib/pd-extended/bin/pd.tk: can't open script
invalid command name "pdtk_post"
invalid command name "pdtk_post"
invalid command name "pdtk_post"
invalid command name "pdtk_pd_startup"

/usr/lib/pd-extended/bin/pd.tk difinetely exists.
Is it my Tcl installation?