[xml] Release of libxml2 2.9.14

[Nick Wellnhofer <wellnhofer aevum de>]

Version 2.9.14 of libxml2 is available at:

    https://download.gnome.org/sources/libxml2/2.9/

Note that starting with 2.9.13, libxml2 tarballs are published on 
download.gnome.org instead of ftp.xmlsoft.org.

### Security

- [CVE-2022-29824] Integer overflow in xmlBuf and xmlBuffer
- Fix potential double-free in xmlXPtrStringRangeFunction
- Fix memory leak in xmlFindCharEncodingHandler
- Normalize XPath strings in-place
- Prevent integer-overflow in htmlSkipBlankChars() and xmlSkipBlankChars()
  (David Kilzer)
- Fix leak of xmlElementContent (David Kilzer)

### Bug fixes

- Fix parsing of subtracted regex character classes
- Fix recursion check in xinclude.c
- Reset last error in xmlCleanupGlobals
- Fix certain combinations of regex range quantifiers
- Fix range quantifier on subregex

### Improvements

- Fix recovery from invalid HTML start tags

### Build system, portability

- Define LFS macros before including system headers
- Initialize XPath floating-point globals
- configure: check for icu DEFS (James Hilliard)
- configure.ac: produce tar.xz only (GNOME policy) (David Seifert)
- CMakeLists.txt: Fix LIBXML_VERSION_NUMBER
- Fix build with older Python versions
- Fix --without-valid build

Thanks to all contributors!

Nick