automake: rework for separated builds

automake: rework for separated builds

Standard practice (including the assumptions of make distcheck) is that
the system can be built in an out-of-tree directory referencing sources
in a read-only filesystem.  The language-specific bindings build
infrastructure, particularly for perl, makes that difficult but
achievable.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>