Define __GNUC_PREREQ if the compiler doesn't.
Define __GNUC_PREREQ if the compiler doesn't.

This fixes the build on MacOS.

fluffy/daala,HeadhunterXamd/daala,vr000m/daala,nvoron23/daala,xiph/daala,KyleSiefring/daala,tribouille/daala,kodabb/daala,ycho/daala,ascent12/daala,tribouille/daala,HeadhunterXamd/daala,tribouille/daala,ycho/daala,ekr/daala,tdaede/daala,vr000m/daala,xiph/daala,mbebenita/daala,metajack/daala,kustom666/daala,tdaede/daala,jmvalin/daala,ascent12/daala,xiph/daala,xiphmont/daala,tdaede/daala,nvoron23/daala,iankronquist/daala,ycho/daala,ascent12/daala,ekr/daala,xiphmont/daala,ycho/daala,kustom666/daala,smarter/daala,nvoron23/daala,iankronquist/daala,metajack/daala,smarter/daala,vr000m/daala,HeadhunterXamd/daala,ycho/daala,jmvalin/daala,jmvalin/daala,xiphmont/daala,felipebetancur/daala,kbara/daala,xiph/daala,metajack/daala,felipebetancur/daala,tdaede/daala_awcy_runs,iankronquist/daala,tribouille/daala,luctrudeau/daala,HeadhunterXamd/daala,tdaede/daala_awcy_runs,kbara/daala,smarter/daala,kustom666/daala,tdaede/daala_awcy_runs,kbara/daala,kodabb/daala,luctrudeau/daala,KyleSiefring/daala,luctrudeau/daala,kustom666/daala,KyleSiefring/daala,xiphmont/daala,xiph/daala,kustom666/daala,kodabb/daala,fluffy/daala,xiphmont/daala,mbebenita/daala,felipebetancur/daala,mbebenita/daala,smarter/daala,nvoron23/daala,mbebenita/daala,luctrudeau/daala,jmvalin/daala,nvoron23/daala,kbara/daala,vr000m/daala,mbebenita/daala,vr000m/daala,HeadhunterXamd/daala,tribouille/daala,tdaede/daala_awcy_runs,KyleSiefring/daala,kodabb/daala,luctrudeau/daala,jmvalin/daala,iankronquist/daala,ekr/daala,felipebetancur/daala,ascent12/daala,iankronquist/daala,KyleSiefring/daala,fluffy/daala,tdaede/daala,kodabb/daala,ascent12/daala,felipebetancur/daala