Hey Ludo,

Then, here's a patch that set the default target to 'current in
lower-object, gexp->file and gexp->script. Then, (current-target-system)
is used at bind time if target is 'current (same way as gexp->derivation
and lower-gexp).

An additionnal patch is needed because lower-object is called without a
default target in lower-gexp, when lowering extensions.

WDYT?

Mathieu