FindLibXml2: Restore compatibility with projects setting LIBXML2_LIBRARIES
In commit a1b38865 (FindLibXml2: Switch to standard module variable names, 2017-07-11) the cache variable used to store the find_library result was renamed. This breaks projects that set the old variable name explicitly. Fix compatibility with such projects by using the old variable as a default if it is set and the new one is not.
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