build: Automake throws a lot of "[...] option 'subdir-objects' is disabled"

[zimmerle]

Automake version 1.4+ is throwing a lot of "[...] option 'subdir-objects' is disabled".

Automake output:

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.
[...]
[...]: warning: source file '$(...)' is in a subdirectory,
[...]: but option 'subdir-objects' is disabled
[...]

From automake "NEWS/ChangeLog":

  - The next major Automake version (2.0) will unconditionally activate
    the 'subdir-objects' option.  In order to smooth out the transition,
    we now give a warning (in the category 'unsupported') whenever a
    source file is present in a subdirectory but the 'subdir-object' is
    not enabled.  For example, the following usage will trigger such a
    warning:

        bin_PROGRAMS = sub/foo
        sub_foo_SOURCES = sub/main.c sub/bar.c