Ewewukek/fix linux build

Fixes building on Linux with: gcc 13.2.1 libxml2 2.12.2

I believe more strict constness for __xmlParserVersion() should not affect compiling with other xml libraries. Also I wasn't sure that including libxml/parser.h works for everyone, so I wrapped it in a #ifdef HAVE_LIBXML2

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Imported from https://github.com/ManaPlus/ManaPlus/pull/41

closes #61 (closed)