[cairo-ft-font.c] conditionalize the changes by 3ec4aa24 on 2018-07-16 for…
[cairo-ft-font.c] conditionalize the changes by 3ec4aa24 on 2018-07-16 for legacy FreeType2 without color font feature.
By 3ec4aa24 , now cairo needs newer FreeType2 with the color font features (at least, the predefined macros for them). This patch is to conditionalize them, for the older FreeType2 users.