[tex4ht] [bug #309] tex4ht does not work with package savetrees

[Karl Berry]

    changed xhlatex calls in the Makefile to mk4ht xhlatex, as xhlatex
    script seems to be missing in TL.

I will reinstate xhlatex in tl.  Darn it, I meant to do before the
release this year.  (I only realized belatedly that it was actually used
all the time in Eitan's builds.)  So our tex4ht Makefile doesn't need to
be changed.  (More on other stuff later.)  --thanks, karl.