This gives users a possibility to deactivate new features not (yet) in the
OpenType standard.
* include/freetype/config/ftoption.h, devel/ftoption.h
(TT_CONFIG_OPTION_NO_BORING_EXPANSION): New macro.
* src/truetype/ttgxvar.c (ft_var_load_avar): Use it to disable 'avar'
version 2.0 support.