As another data point, we released the Jessie-based MX 16 Linux with libfreetype6 2.7 and the modified fontconfig from this thread, and there have been zero problems with it.
I'll have to check to see how Debian configured their build, though.
The MX Linux repositories
: Backports galore! If we don't have something, just ask and we'll try--we like challenges. New packages: AzPainter 2.0.4, Pale Moon 27.2.1, Liquorix kernel 4.10-2, mpv 0.24, Kodi 17.1, Ksnip 1.3.1, Mesa 13.0.6