Debian removed the `freetype-config` utility in 2.8.1-1 from 2018:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=870618

This patch falls back if `freetype-config` is not available, using flag
values that work on my Debian-based system.

wchargin-branch: freetype-flags
wchargin-source: 470b23f596973e106ea6363c1f96fdc580f45152