x11-toolkits/ocaml-labltk: stop guessing Tcl/Tk version and be verbose

When specific Tcl/Tk definitions are not passed to the configure script,
it tries to guess installed version, header paths, and the libraries.
This routine is not robust and can easily brake in custom environments
or in the future. Pass the correct values conveniently offered by our
framework instead; this would also slightly speed up configure process.

While here, do not pass ${STAGEDIR} via MAKE_ENV, it is already passed
on the MAKE_ARGS (as DESTDIR). Adjust the port's Makefile accordingly.