gnome.py: put external_ldflags before internal
gnome.py: put external_ldflags before internal

Reason is GLib. GLib will (always?) be an external dependency, and it is
is crucial that this is *our* GLib, and not the system GLib.

If the internal flags includes some system-libraries, they might add -L
options to /opt/local or /usr/lib that will make the scanner pick up
a system-installed GLib first.

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