Since e5687602 it's instance
responsability to generate a fonts.conf if they need fonts, but we
realized to late that if there is no /etc/fonts/fonts.conf on the
machine firefox does not start.

In recent ERP5 software release, we have a $FONTCONFIG_FILE, but on
older versions we don't.

To keep compatibility with old ERP5 software release, we generate a
default FONTCONFIG_FILE that will be used as fallback when
$FONTCONFIG_FILE environment variable is not set.