ttf-dejavu 2.37 will change the way fontconfig configuration is installed. In previous versions the configuration was symlinked from post_install/post_upgrade, the new version will place the files inside the package like it is done in fontconfig now.
For more information about this change: https://bugs.archlinux.org/task/32312
To upgrade to ttf-dejavu 2.37 it's recommended to upgrade the package on its own:
pacman -S --force ttf-dejavu