Here's a first pass at turning the ZLE_UNICODE_SUPPORT define into a
configure item.  The attached patch allows the user to specify either
--with-unicode-support or --without-unicode-support (to force the
decision), otherwise it runs a test to see if it thinks unicode can
be supported (which consists of the current checks from system.h
transplanted into configure).  The code in system.h was also modified
to check the value of ZLE_UNICODE_SUPPORT instead of set it.

Some questions that should be answered:

  o Do we like the above name for the --with/--without option?
  o Is including locale.h (if it exists) the right include file for
    the new configure test for __STDC_ISO_10646__?  (It works on my
    Linux system.)
  o Do we need the old unicode "subset" logic that I removed from
    system.h?

..wayne..