[build] ./configure --enable-werror: add unused-variable
[build] ./configure --enable-werror: add unused-variable

Turn corresponding warning on by default (not always covered by -Wall).

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