configure.ac: fork() during registry scanning is unsafe on Cygwin
configure.ac: fork() during registry scanning is unsafe on Cygwin

Fixes #555978.

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