added mixer example and adder plugin to configure.in
added mixer example and adder plugin to configure.in

Original commit message from CVS:
added mixer example and adder plugin to configure.in

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