Update to 2.5.3

Changelog:
Overview of changes leading to 2.5.3
Wednesday, June 26, 2019
====================================
- Fix UCD script data for Unicode 10+ scripts.  This was broken since 2.5.0.
- More optimizations for HB_TINY.

Overview of changes leading to 2.5.2
Thursday, June 20, 2019
====================================
- More hb-config.hh facilities to shrink library size, namely when built as
  HB_TINY.
- New documentation of custom configurations in CONFIG.md.
- Fix build on gcc 4.8.  That's supported again.
- Universal Shaping Engine improvements thanks to David Corbett.
- API Changes: Undeprecate some horizontal-kerning API and re-enable in hb-ft,
  such that Type1 fonts will continue kerning.

Overview of changes leading to 2.5.1
Friday, May 31, 2019
====================================
- Fix build with various versions of Visual Studio.
- Improved documentation, thanks to Nathan Willis.
- Bugfix in subsetting glyf table.
- Improved scripts for cross-compiling for Windows using mingw.
- Rename HB_MATH_GLYPH_PART_FLAG_EXTENDER to HB_OT_MATH_GLYPH_PART_FLAG_EXTENDER.
  A deprecated macro is added for backwards-compatibility.

Overview of changes leading to 2.5.0
Friday, May 24, 2019
====================================
- This release does not include much functional changes, but includes major internal
  code-base changes.  We now require C++11.  Support for gcc 4.8 and earlier has been
  dropped.
- New hb-config.hh facility for compiling smaller library for embedded and web usecases.
- New Unicode Character Databse implementation that is half the size of previously-used
  UCDN.
- Subsetter improvements.
- Improved documentation, thanks to Nathan Willis.
- Misc shaping fixes.


(ryoon)