Always try to read distro-configs/default.conf (if existing) before
any option parsing.

That way, downstreams (distros, etc) can just place there site config
into the tree without having to pass any additional options to autogen.sh,
and even automatic invocations will always have the right parameters.

Change-Id: Ic5bf68adc719476d374cf03e31e054b69c931b72
Reviewed-on: https://gerrit.libreoffice.org/1096Reviewed-by: Andras Timar <atimar@suse.com>
Tested-by: Andras Timar <atimar@suse.com>