automake: use subdir-objects option

automake: use subdir-objects option

Automake 1.14 introduced several non-fatal warnings that should help
projects prepare to the next major automake release (2.0).

Considering the way OpenOCD automake files are written, using
subdir-objects doesn't have any adverse effects, so enable it for the
future compatibility.

Change-Id: I3e7fd93d1b53c5a7ed00ec0f03d2d1510a07f516
Reported-by: Freddie Chopin <freddie_chopin@op.pl>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1517
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>