place to change if you need to use a different Tcl/Tk version.
- provide MODTCL_LIB and MODTK_LIB (avoids a possible messy
construct in an individual port's Makefile when they are needed,
allows use of "LDFLAGS=-L${MODTCL_LIBDIR} -l${MODTCL_LIB}").
ok steven@, Stuart Cassoff