configure.ac: enable automake's subdir-objects option

configure.ac: enable automake's subdir-objects option

From [1]:

    subdir-objects

        If this option is specified, then objects are placed into the
        subdirectory of the build directory corresponding to the
        subdirectory of the source file. For instance, if the source file is
        subdir/file.cxx, then the output file would be subdir/file.o. See
        Program and Library Variables.

This will allow reducing the number of Makefiles, but placing rules in
Makefiles in parent directories, instead of having Makefiles in every
single directory with something that needs to be built.

[1] https://www.gnu.org/software/automake/manual/html_node/List-of-Automake-options.html

Change-Id: Ic0c6ee5d809dee6b9e560239abd15408b28d9695
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>