Trivial: Use g_clear_object more

It no longer has atomic overhead, so we can use it everywhere.
Updates for 3.13.6
Avoid "pkg-config --atleast-version"

AM_PATH_GTK_3_0([3.4], [], [], [gthread]) results in calling
"pkg-config --atleast-version 3.4 gtk+-3.0 gthread-2.0".

With pkg-config 0.28, that test fails, because the version of
gthread-2.0 is smaller than 3.4. This appears to be a behaviour
change since 0.26, but matches what the documentation said in 0.26.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=641638
Bug-Debian: https://bugs.debian.org/756476
Reviewed-by: Matthias Clasen <mclasen@redhat.com>