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?