[xslt] Release of libxslt 1.1.37

[Nick Wellnhofer <wellnhofer aevum de>]

Version 1.1.37 of libxslt is available at:

    https://download.gnome.org/sources/libxslt/1.1/

### Improvements

- Don't use deprecated libxml2 macros
- Don't mess with xmlDefaultSAXHandler

### Build system

- Require automake-1.16.3 or later
- Remove generated files from distribution
- Add missing compile definition for static builds to Autotools (Mike Dalessio)

Nick