Port to glib 2.31.x g_thread API

Port to glib 2.31.x g_thread API

g_thread_init() is deprecated since glib 2.24, call g_type_init() instead.
Bump glib requirement accordingly.

g_thread_create is deprecated since 2.31, use g_thread_new() instead. When
building with a glib earlier than 2.31, provide a backwards compatibility shim.

[Added a comment about why we're using g_type_init() in a test that
doesn't otherwise use GObject -smcv]

[Applied to 1.4 despite just being a deprecation fix because it also fixes
linking with GLib 2.32, in which gthread has been removed from gobject's
Requires and moved to Requires.private, Debian #665665 -smcv]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=44413
Bug-Debian: http://bugs.debian.org/665665
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>