build: enable -Werror=gnu
build: enable -Werror=gnu

Stop the build if a warning is emitted due to `-Wgnu` and
`--enable-werror` has been used. As usual - this would help notice such
a warning that is about to be introduced in new code.

This is a followup to
https://github.com/bitcoin/bitcoin/pull/18088
build: ensure we aren't using GNU extensions

achow101/bitcoin,achow101/bitcoin,andreaskern/bitcoin,pstratem/bitcoin,MeshCollider/bitcoin,Xekyo/bitcoin,tecnovert/particl-core,Sjors/bitcoin,MarcoFalke/bitcoin,mruddy/bitcoin,jamesob/bitcoin,yenliangl/bitcoin,qtumproject/qtum,dscotese/bitcoin,jonasschnelli/bitcoin,midnightmagic/bitcoin,jambolo/bitcoin,EthanHeilman/bitcoin,Xekyo/bitcoin,fujicoin/fujicoin,midnightmagic/bitcoin,sstone/bitcoin,AkioNak/bitcoin,particl/particl-core,dscotese/bitcoin,prusnak/bitcoin,GroestlCoin/GroestlCoin,MeshCollider/bitcoin,bitcoinsSG/bitcoin,ElementsProject/elements,jamesob/bitcoin,JeremyRubin/bitcoin,yenliangl/bitcoin,andreaskern/bitcoin,ElementsProject/elements,MeshCollider/bitcoin,AkioNak/bitcoin,prusnak/bitcoin,GroestlCoin/bitcoin,particl/particl-core,pstratem/bitcoin,instagibbs/bitcoin,MeshCollider/bitcoin,rnicoll/dogecoin,anditto/bitcoin,sstone/bitcoin,Xekyo/bitcoin,bitcoinsSG/bitcoin,fujicoin/fujicoin,fanquake/bitcoin,qtumproject/qtum,yenliangl/bitcoin,tecnovert/particl-core,apoelstra/bitcoin,fanquake/bitcoin,tecnovert/particl-core,namecoin/namecore,namecoin/namecore,domob1812/namecore,jlopp/statoshi,JeremyRubin/bitcoin,fujicoin/fujicoin,EthanHeilman/bitcoin,jambolo/bitcoin,ajtowns/bitcoin,Xekyo/bitcoin,n1bor/bitcoin,qtumproject/qtum,bitcoinsSG/bitcoin,GroestlCoin/GroestlCoin,sipsorcery/bitcoin,GroestlCoin/bitcoin,rnicoll/dogecoin,pataquets/namecoin-core,midnightmagic/bitcoin,EthanHeilman/bitcoin,bitcoin/bitcoin,anditto/bitcoin,domob1812/namecore,bitcoinknots/bitcoin,dscotese/bitcoin,MeshCollider/bitcoin,sipsorcery/bitcoin,fujicoin/fujicoin,apoelstra/bitcoin,rnicoll/bitcoin,achow101/bitcoin,andreaskern/bitcoin,domob1812/namecore,jnewbery/bitcoin,Xekyo/bitcoin,apoelstra/bitcoin,lateminer/bitcoin,andreaskern/bitcoin,n1bor/bitcoin,litecoin-project/litecoin,jambolo/bitcoin,mruddy/bitcoin,rnicoll/dogecoin,domob1812/namecore,andreaskern/bitcoin,jamesob/bitcoin,MarcoFalke/bitcoin,domob1812/bitcoin,kallewoof/bitcoin,lateminer/bitcoin,pstratem/bitcoin,dscotese/bitcoin,fujicoin/fujicoin,rnicoll/bitcoin,ajtowns/bitcoin,pataquets/namecoin-core,practicalswift/bitcoin,cdecker/bitcoin,JeremyRubin/bitcoin,alecalve/bitcoin,sstone/bitcoin,jlopp/statoshi,bitcoinknots/bitcoin,prusnak/bitcoin,bitcoinsSG/bitcoin,particl/particl-core,prusnak/bitcoin,apoelstra/bitcoin,jnewbery/bitcoin,anditto/bitcoin,practicalswift/bitcoin,mm-s/bitcoin,bitcoin/bitcoin,cdecker/bitcoin,namecoin/namecoin-core,n1bor/bitcoin,litecoin-project/litecoin,apoelstra/bitcoin,namecoin/namecoin-core,sipsorcery/bitcoin,jlopp/statoshi,mm-s/bitcoin,bitcoinknots/bitcoin,kallewoof/bitcoin,MarcoFalke/bitcoin,Sjors/bitcoin,pataquets/namecoin-core,bitcoinsSG/bitcoin,jamesob/bitcoin,practicalswift/bitcoin,yenliangl/bitcoin,cdecker/bitcoin,dscotese/bitcoin,tecnovert/particl-core,namecoin/namecoin-core,kallewoof/bitcoin,particl/particl-core,fanquake/bitcoin,particl/particl-core,namecoin/namecoin-core,mruddy/bitcoin,instagibbs/bitcoin,pataquets/namecoin-core,tecnovert/particl-core,alecalve/bitcoin,Sjors/bitcoin,MeshCollider/bitcoin,namecoin/namecoin-core,AkioNak/bitcoin,JeremyRubin/bitcoin,ajtowns/bitcoin,qtumproject/qtum,cdecker/bitcoin,mm-s/bitcoin,EthanHeilman/bitcoin,jnewbery/bitcoin,domob1812/bitcoin,lateminer/bitcoin,kallewoof/bitcoin,jnewbery/bitcoin,namecoin/namecore,fanquake/bitcoin,mruddy/bitcoin,practicalswift/bitcoin,GroestlCoin/GroestlCoin,Sjors/bitcoin,AkioNak/bitcoin,rnicoll/bitcoin,alecalve/bitcoin,namecoin/namecore,fujicoin/fujicoin,namecoin/namecore,sipsorcery/bitcoin,litecoin-project/litecoin,JeremyRubin/bitcoin,ajtowns/bitcoin,cdecker/bitcoin,alecalve/bitcoin,lateminer/bitcoin,AkioNak/bitcoin,sstone/bitcoin,kallewoof/bitcoin,cdecker/bitcoin,anditto/bitcoin,namecoin/namecoin-core,n1bor/bitcoin,anditto/bitcoin,domob1812/bitcoin,n1bor/bitcoin,jambolo/bitcoin,jlopp/statoshi,litecoin-project/litecoin,EthanHeilman/bitcoin,lateminer/bitcoin,bitcoin/bitcoin,jambolo/bitcoin,ElementsProject/elements,anditto/bitcoin,apoelstra/bitcoin,mm-s/bitcoin,rnicoll/bitcoin,ElementsProject/elements,alecalve/bitcoin,jonasschnelli/bitcoin,GroestlCoin/GroestlCoin,instagibbs/bitcoin,domob1812/bitcoin,rnicoll/bitcoin,sipsorcery/bitcoin,GroestlCoin/bitcoin,GroestlCoin/GroestlCoin,ajtowns/bitcoin,mruddy/bitcoin,EthanHeilman/bitcoin,mm-s/bitcoin,MarcoFalke/bitcoin,ElementsProject/elements,sipsorcery/bitcoin,bitcoinknots/bitcoin,rnicoll/bitcoin,rnicoll/dogecoin,rnicoll/dogecoin,Sjors/bitcoin,achow101/bitcoin,jnewbery/bitcoin,jonasschnelli/bitcoin,jlopp/statoshi,pstratem/bitcoin,jamesob/bitcoin,bitcoin/bitcoin,achow101/bitcoin,domob1812/bitcoin,particl/particl-core,qtumproject/qtum,GroestlCoin/bitcoin,prusnak/bitcoin,yenliangl/bitcoin,andreaskern/bitcoin,AkioNak/bitcoin,practicalswift/bitcoin,GroestlCoin/bitcoin,Xekyo/bitcoin,pataquets/namecoin-core,pataquets/namecoin-core,mm-s/bitcoin,achow101/bitcoin,n1bor/bitcoin,bitcoinknots/bitcoin,ajtowns/bitcoin,midnightmagic/bitcoin,lateminer/bitcoin,instagibbs/bitcoin,midnightmagic/bitcoin,tecnovert/particl-core,MarcoFalke/bitcoin,jambolo/bitcoin,sstone/bitcoin,domob1812/namecore,MarcoFalke/bitcoin,qtumproject/qtum,bitcoin/bitcoin,dscotese/bitcoin,yenliangl/bitcoin,litecoin-project/litecoin,kallewoof/bitcoin,pstratem/bitcoin,fanquake/bitcoin,qtumproject/qtum,prusnak/bitcoin,practicalswift/bitcoin,ElementsProject/elements,GroestlCoin/bitcoin,GroestlCoin/GroestlCoin,jonasschnelli/bitcoin,JeremyRubin/bitcoin,domob1812/bitcoin,pstratem/bitcoin,bitcoinsSG/bitcoin,midnightmagic/bitcoin,jlopp/statoshi,jonasschnelli/bitcoin,instagibbs/bitcoin,instagibbs/bitcoin,alecalve/bitcoin,jamesob/bitcoin,namecoin/namecore,fanquake/bitcoin,litecoin-project/litecoin,domob1812/namecore,bitcoin/bitcoin,mruddy/bitcoin,sstone/bitcoin