Silence subdir-objects warning from automake 1.14 (#781978)

Configuring GParted on a machine with automake >= 1.14 produces this
warning:

    $ ./autogen.sh
    ...
    lib/gtest/Makefile.am:58: warning: source file 'src/gtest-all.cc' is in a subdirectory,
    lib/gtest/Makefile.am:58: but option 'subdir-objects' is disabled
    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.
    lib/gtest/Makefile.am:60: warning: source file 'src/gtest_main.cc' is in a subdirectory,
    lib/gtest/Makefile.am:60: but option 'subdir-objects' is disabled
    lib/gtest/Makefile.am: installing './depcomp'
    tests/Makefile.am:22: warning: source file '../src/BlockSpecial.cc' is in a subdirectory,
    tests/Makefile.am:22: but option 'subdir-objects' is disabled
    tests/Makefile.am:23: warning: source file '../src/PipeCapture.cc' is in a subdirectory,
    tests/Makefile.am:23: but option 'subdir-objects' is disabled
    ...

From the automake 1.14 NEWS file:
    http://git.savannah.gnu.org/cgit/automake.git/tree/NEWS?h=v1.14#n125

    "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
    "

Set the subdir-objects automake option, silencing the warning.

Bug 781978 - Add Google Test C++ test framework