Libraries flags (like -lglib-2.0) should be provided in the end of
compile command. It's not how it's done right now in tests/Makefile.am,
because _LDFLAGS is used where we should actually use _LDADD. Due to
this, on Ubuntu 16.04 with GCC 5 we have next linking errors:

    gcc -lxcb-damage -lxcb-xfixes -lxcb-render -lxcb-shape -lxcb
        -lxcb-xtest -lxcb -lxcb-shm -lxcb -lxcb-util -lxcb -lgtk-x11-2.0
        -lgdk-x11-2.0 -lpangocairo-1.0 -latk-1.0 -lcairo -lgdk_pixbuf-2.0
        -lgio-2.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lglib-2.0
        -lfontconfig -lfreetype -lspice-server -lglib-2.0 -lpixman-1
        -o x11spice_test tests.o x11spice_test.o xcb.o xdummy.o util.o
        main.o  -laudit

    tests.o: In function `check_binary':
    src/tests/tests.c:83: undefined reference to `g_log'
    src/tests/tests.c:84: undefined reference to `g_test_skip'
    tests.o: In function `test_common_start':
    src/tests/tests.c:46: undefined reference to `g_test_build_filename'
    src/tests/tests.c:56: undefined reference to `g_log'
    src/tests/tests.c:57: undefined reference to `g_test_fail'
    src/tests/tests.c:39: undefined reference to `g_test_skip'
    src/tests/tests.c:48: undefined reference to `g_log'
    src/tests/tests.c:49: undefined reference to `g_test_fail'

Use _LDADD instead of _LDFLAGS for libs, so that libraries are
specified in the end of compile line.

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>