The fontconfig 2.10.1 update overwrites symlinks created by the former package version. These symlinks need to be removed before the update:
rm /etc/fonts/conf.d/20-unhint-small-vera.conf
rm /etc/fonts/conf.d/20-fix-globaladvance.conf
rm /etc/fonts/conf.d/29-replace-bitmap-fonts.conf
rm /etc/fonts/conf.d/30-metric-aliases.conf
rm /etc/fonts/conf.d/30-urw-aliases.conf
rm /etc/fonts/conf.d/40-nonlatin.conf
rm /etc/fonts/conf.d/45-latin.conf
rm /etc/fonts/conf.d/49-sansserif.conf
rm /etc/fonts/conf.d/50-user.conf
rm /etc/fonts/conf.d/51-local.conf
rm /etc/fonts/conf.d/60-latin.conf
rm /etc/fonts/conf.d/65-fonts-persian.conf
rm /etc/fonts/conf.d/65-nonlatin.conf
rm /etc/fonts/conf.d/69-unifont.conf
rm /etc/fonts/conf.d/80-delicious.conf
rm /etc/fonts/conf.d/90-synthetic.conf
pacman -Sy fontconfig
Main systemwide configuration should be done by symlinks (especially for autohinting, sub-pixel and lcdfilter):
cd /etc/fonts/conf.d
ln -s ../conf.avail/XX-foo.conf
Also check Font Configuration and Fonts.