Log Message:
libxml2: Update to 2.9.7

- Portability:
  Change preprocessor OS tests to __linux__ (Nick Wellnhofer)

- Bug Fixes:
  Fix XPath stack frame logic (Nick Wellnhofer),
  Report undefined XPath variable error message (Nick Wellnhofer),
  Fix regression with librsvg (Nick Wellnhofer),
  Handle more invalid entity values in recovery mode (Nick Wellnhofer),
  Fix structured validation errors (Nick Wellnhofer),
  Fix memory leak in LZMA decompressor (Nick Wellnhofer),
  Set memory limit for LZMA decompression (Nick Wellnhofer),
  Handle illegal entity values in recovery mode (Nick Wellnhofer),
  Fix debug dump of streaming XPath expressions (Nick Wellnhofer),
  Fix memory leak in nanoftp (Nick Wellnhofer),
  Fix memory leaks in SAX1 parser (Nick Wellnhofer)