Comment 165 for bug 1160569

Created attachment 8630422
Replace g_slice_set_config() with G_SLICE environment variable

Turns out there aren't many places where g_thread_init is actually called. There's some webrtc code that we don't build, and other than that, other uses are exclusive... but need to be factored in, so this became a bit awful. At the same time I added a compile time glib check that removes the g_thread_init call when building against glib >= 2.32, which is the case for gtk3 builds.