configure: bump GLib requirement to 2.18
configure: bump GLib requirement to 2.18

Bump GLib requirement as per the release planning docs.

jpakkane/gstreamer,ylatuya/gstreamer,centricular/gstreamer,krieger-od/gstreamer,collects/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,mparis/gstreamer,ylatuya/gstreamer,cablelabs/gstreamer,drothlis/gstreamer,centricular/gstreamer,cablelabs/gstreamer,drothlis/gstreamer,Lachann/gstreamer,lubosz/gstreamer,mrchapp/gstreamer,cfoch/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,Distrotech/gstreamer,StreamUtils/gstreamer,ylatuya/gstreamer,magcius/gstreamer,mparis/gstreamer,cfoch/gstreamer,magcius/gstreamer,jpakkane/gstreamer,mparis/gstreamer,Distrotech/gstreamer,magcius/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,krichter722/gstreamer,Lachann/gstreamer,krieger-od/gstreamer,ylatuya/gstreamer,StreamUtils/gstreamer,ensonic/gstreamer,drothlis/gstreamer,mparis/gstreamer,jpxiong/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,collects/gstreamer,justinjoy/gstreamer,jpakkane/gstreamer,collects/gstreamer,collects/gstreamer,ylatuya/gstreamer,ahmedammar/platform_external_gst_gstreamer,cfoch/gstreamer,jpakkane/gstreamer,magcius/gstreamer,centricular/gstreamer,Distrotech/gstreamer,lubosz/gstreamer,ensonic/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,Distrotech/gstreamer,drothlis/gstreamer,mrchapp/gstreamer,ahmedammar/platform_external_gst_gstreamer,mrchapp/gstreamer,cfoch/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,jpxiong/gstreamer,justinjoy/gstreamer,ensonic/gstreamer,centricular/gstreamer,krichter722/gstreamer,ahmedammar/platform_external_gst_gstreamer,surround-io/gstreamer,collects/gstreamer,mrchapp/gstreamer,StreamUtils/gstreamer,justinjoy/gstreamer,jpxiong/gstreamer,mrchapp/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,surround-io/gstreamer,lovebug356/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,lovebug356/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,Distrotech/gstreamer,mparis/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,magcius/gstreamer,lubosz/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,ahmedammar/platform_external_gst_gstreamer,lovebug356/gstreamer,Lachann/gstreamer,Lachann/gstreamer,ensonic/gstreamer,krichter722/gstreamer,drothlis/gstreamer,ahmedammar/platform_external_gst_gstreamer,cfoch/gstreamer