feature request: Add support for XDG_DATA_DIRS
The XDG base directory specification (https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html) defines the XDG_DATA_DIRS as "a set of preference ordered base directories relative to which data files should be searched".
fontconfig currently only supports the XDG_DATA_HOME XDG environment variable, which is user-specific and can only include a single location.
Honoring XDG_DATA_DIRS would allow for fonts discovery on system where following the File Hierarchy Standard (FHS) is not practical (such as GNU Guix, NixOS, etc.)
Thank you for your consideration!