`configure' change for libxml

[Jonathan Gapen]

    I just committed to CVS another change to the libxml detection in
the `configure' scripts.  Libxml follows the trend of including a script
that makefiles and configure scripts can run to find out what flags to
use to link against the library.  The latest libxml distributions
include an autoconf macro which tries to use this script to detect
libxml.  I've integrated that into the GNUstep.
    It's more flexible, less hack-ish, more consistent with other
software packages, and probably more reliable than the previous method. 
It does mean a change to the `configure' arguments:  now there's
--with-xml-prefix=PATH and --disable-xmltest.  It also may be necessary
on non-standard installs to set the environment variable XML_CONFIG to
the full path of the xml-config script, which on FreeBSD happens to be
/usr/local/bin/xml2-config.  (This is SOP for any package that uses
libxml, GTK+, glib, and SDL from the FreeBSD packages collection, so it
shouldn't be unfamiliar to users.)