Use subdir-objects Automake option

If you don't pass subdir-objects, Automake 1.14+ will complain loudly
and fail:

[...]
automake: warnings are treated as errors
tests/media/101greatgoals.mk:2: warning: source file
'media/media_101greatgoals.c' is in a subdirectory,
tests/media/101greatgoals.mk:2: but option 'subdir-objects' is disabled
tests/Makefile.am:21:   'tests/media/tests.mk' included from here
tests/media/tests.mk:1:   'tests/media/101greatgoals.mk' included from
here
automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.
tests/media/ardmediathek.mk:2: warning: source file
'media/media_ardmediathek.c' is in a subdirectory,
tests/media/ardmediathek.mk:2: but option 'subdir-objects' is disabled
[...]

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>