Given the specified external entity name, return the associated filename.
This provides applications a way to convert various configuration file
references into filename form.
A null or empty name indicates that the default configuration file should
be used; which file this references can be overridden with the
FONTCONFIG_FILE environment variable. Next, if the name starts with ~, it
refers to a file in the current users home directory. Otherwise if the name
doesnt start with /, it refers to a file in the default configuration
directory; the built-in default directory can be overridden with the
FONTCONFIG_PATH environment variable.