Use subdir-objects option in Automake

automake 1.14 showes the following warning:

    test/unit/Makefile.am:24: warning: source file '../../mrn_sys.cpp' is in a subdirectory,
    test/unit/Makefile.am:24: but option 'subdir-objects' is disabled
    automake-1.14: warning: possible forward-incompatibility.
    automake-1.14: At least a source file is in a subdirectory, but the 'subdir-objects'
    automake-1.14: automake option hasn't been enabled.  For now, the corresponding output
    automake-1.14: object file(s) will be placed in the top-level directory.  However,
    automake-1.14: this behaviour will change in future Automake versions: they will
    automake-1.14: unconditionally cause object files to be placed in the same subdirectory
    automake-1.14: of the corresponding sources.
    automake-1.14: You are advised to start using 'subdir-objects' option throughout your
    automake-1.14: project, to avoid future incompatibilities.

So I use subdir-objects option.

See:
http://www.gnu.org/software/automake/manual/automake.html#index-subdir_002dobjects