Update to latest API changes in GLib/GIO and require at least gio-2.0 2.15.0 for...

Update to latest API changes in GLib/GIO and require at least gio-2.0 2.15.0 for this.

Original commit message from CVS:
* configure.ac:
* ext/gio/gstgio.c:
* ext/gio/gstgio.h:
* ext/gio/gstgiobasesink.h:
* ext/gio/gstgiobasesrc.c: (gst_gio_base_src_get_size):
* ext/gio/gstgiobasesrc.h:
* ext/gio/gstgiosink.c: (gst_gio_sink_start):
* ext/gio/gstgiosink.h:
* ext/gio/gstgiosrc.h:
* ext/gio/gstgiostreamsink.h:
* ext/gio/gstgiostreamsrc.h:
* tests/check/pipelines/gio.c:
Update to latest API changes in GLib/GIO and require at least
gio-2.0 2.15.0 for this.
* ext/gio/Makefile.am:
Add GST_PLUGIN_LDFLAGS to LDFLAGS.