contrib: guix: More robust search paths, add checks
contrib: guix: More robust search paths, add checks

- store_path() previously only worked for cross compilation packages, we
  remove this assumption here
- Add CROSS_GCC_LIB variable which points to where gcc libs/headers are
  located
- Add gcc libs/headers to our CROSS_*_PATH environment variables
- Check that all directories in CROSS_*_PATH are sane

kallewoof/bitcoin,bitcoinsSG/bitcoin,mm-s/bitcoin,mruddy/bitcoin,namecoin/namecoin-core,pstratem/bitcoin,jamesob/bitcoin,ajtowns/bitcoin,andreaskern/bitcoin,mitchellcash/bitcoin,practicalswift/bitcoin,fujicoin/fujicoin,tecnovert/particl-core,sstone/bitcoin,bitcoinknots/bitcoin,lateminer/bitcoin,alecalve/bitcoin,midnightmagic/bitcoin,wellenreiter01/Feathercoin,GroestlCoin/GroestlCoin,GroestlCoin/GroestlCoin,MeshCollider/bitcoin,lateminer/bitcoin,FeatherCoin/Feathercoin,litecoin-project/litecoin,yenliangl/bitcoin,namecoin/namecore,jambolo/bitcoin,anditto/bitcoin,bitcoin/bitcoin,rnicoll/bitcoin,domob1812/namecore,JeremyRubin/bitcoin,Sjors/bitcoin,n1bor/bitcoin,namecoin/namecore,vertcoin/vertcoin,apoelstra/bitcoin,rnicoll/bitcoin,cdecker/bitcoin,mitchellcash/bitcoin,fujicoin/fujicoin,pstratem/bitcoin,jamesob/bitcoin,nikkitan/bitcoin,particl/particl-core,sipsorcery/bitcoin,alecalve/bitcoin,DigitalPandacoin/pandacoin,ahmedbodi/vertcoin,rnicoll/dogecoin,cdecker/bitcoin,rnicoll/dogecoin,wellenreiter01/Feathercoin,peercoin/peercoin,nikkitan/bitcoin,domob1812/bitcoin,monacoinproject/monacoin,mm-s/bitcoin,bitcoin/bitcoin,namecoin/namecore,instagibbs/bitcoin,n1bor/bitcoin,n1bor/bitcoin,practicalswift/bitcoin,anditto/bitcoin,jtimon/bitcoin,FeatherCoin/Feathercoin,mitchellcash/bitcoin,Sjors/bitcoin,Xekyo/bitcoin,ahmedbodi/vertcoin,GroestlCoin/bitcoin,jtimon/bitcoin,FeatherCoin/Feathercoin,fujicoin/fujicoin,jonasschnelli/bitcoin,sipsorcery/bitcoin,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,jtimon/bitcoin,domob1812/bitcoin,jnewbery/bitcoin,kallewoof/bitcoin,tecnovert/particl-core,pataquets/namecoin-core,DigitalPandacoin/pandacoin,achow101/bitcoin,Sjors/bitcoin,sipsorcery/bitcoin,qtumproject/qtum,jamesob/bitcoin,jambolo/bitcoin,apoelstra/bitcoin,pstratem/bitcoin,pstratem/bitcoin,achow101/bitcoin,AkioNak/bitcoin,monacoinproject/monacoin,yenliangl/bitcoin,gjhiggins/vcoincore,nikkitan/bitcoin,jtimon/bitcoin,prusnak/bitcoin,apoelstra/bitcoin,peercoin/peercoin,midnightmagic/bitcoin,prusnak/bitcoin,EthanHeilman/bitcoin,pataquets/namecoin-core,jlopp/statoshi,pataquets/namecoin-core,ahmedbodi/vertcoin,qtumproject/qtum,jonasschnelli/bitcoin,lateminer/bitcoin,ahmedbodi/vertcoin,cdecker/bitcoin,Xekyo/bitcoin,anditto/bitcoin,instagibbs/bitcoin,Xekyo/bitcoin,achow101/bitcoin,practicalswift/bitcoin,particl/particl-core,bitcoin/bitcoin,apoelstra/bitcoin,monacoinproject/monacoin,jonasschnelli/bitcoin,particl/particl-core,tjps/bitcoin,ajtowns/bitcoin,EthanHeilman/bitcoin,sstone/bitcoin,dscotese/bitcoin,GroestlCoin/bitcoin,domob1812/namecore,MarcoFalke/bitcoin,MarcoFalke/bitcoin,instagibbs/bitcoin,pataquets/namecoin-core,fujicoin/fujicoin,n1bor/bitcoin,jonasschnelli/bitcoin,litecoin-project/litecoin,sipsorcery/bitcoin,mruddy/bitcoin,jlopp/statoshi,litecoin-project/litecoin,sstone/bitcoin,anditto/bitcoin,namecoin/namecore,wellenreiter01/Feathercoin,fanquake/bitcoin,midnightmagic/bitcoin,FeatherCoin/Feathercoin,jlopp/statoshi,mm-s/bitcoin,litecoin-project/litecoin,GroestlCoin/bitcoin,pataquets/namecoin-core,vertcoin/vertcoin,dscotese/bitcoin,tecnovert/particl-core,Xekyo/bitcoin,FeatherCoin/Feathercoin,bitcoinknots/bitcoin,mitchellcash/bitcoin,dscotese/bitcoin,practicalswift/bitcoin,bitcoin/bitcoin,GroestlCoin/GroestlCoin,FeatherCoin/Feathercoin,domob1812/namecore,anditto/bitcoin,MeshCollider/bitcoin,AkioNak/bitcoin,n1bor/bitcoin,JeremyRubin/bitcoin,domob1812/bitcoin,apoelstra/bitcoin,lateminer/bitcoin,CryptArc/bitcoin,domob1812/namecore,pstratem/bitcoin,prusnak/bitcoin,MeshCollider/bitcoin,qtumproject/qtum,tjps/bitcoin,monacoinproject/monacoin,ahmedbodi/vertcoin,wellenreiter01/Feathercoin,MarcoFalke/bitcoin,ajtowns/bitcoin,MarcoFalke/bitcoin,rnicoll/bitcoin,tjps/bitcoin,mruddy/bitcoin,jambolo/bitcoin,nikkitan/bitcoin,prusnak/bitcoin,n1bor/bitcoin,bitcoinsSG/bitcoin,prusnak/bitcoin,alecalve/bitcoin,mruddy/bitcoin,ElementsProject/elements,jlopp/statoshi,ajtowns/bitcoin,monacoinproject/monacoin,instagibbs/bitcoin,lateminer/bitcoin,domob1812/bitcoin,andreaskern/bitcoin,ajtowns/bitcoin,jlopp/statoshi,sipsorcery/bitcoin,tecnovert/particl-core,mm-s/bitcoin,namecoin/namecoin-core,qtumproject/qtum,nikkitan/bitcoin,peercoin/peercoin,AkioNak/bitcoin,mitchellcash/bitcoin,DigitalPandacoin/pandacoin,jamesob/bitcoin,MarcoFalke/bitcoin,mm-s/bitcoin,yenliangl/bitcoin,yenliangl/bitcoin,instagibbs/bitcoin,mm-s/bitcoin,dscotese/bitcoin,CryptArc/bitcoin,GroestlCoin/bitcoin,particl/particl-core,namecoin/namecoin-core,nikkitan/bitcoin,bitcoinsSG/bitcoin,instagibbs/bitcoin,jnewbery/bitcoin,CryptArc/bitcoin,andreaskern/bitcoin,vertcoin/vertcoin,andreaskern/bitcoin,qtumproject/qtum,ElementsProject/elements,vertcoin/vertcoin,wellenreiter01/Feathercoin,mruddy/bitcoin,rnicoll/bitcoin,gjhiggins/vcoincore,DigitalPandacoin/pandacoin,bitcoin/bitcoin,CryptArc/bitcoin,JeremyRubin/bitcoin,achow101/bitcoin,MeshCollider/bitcoin,alecalve/bitcoin,JeremyRubin/bitcoin,jnewbery/bitcoin,jamesob/bitcoin,monacoinproject/monacoin,particl/particl-core,ElementsProject/elements,cdecker/bitcoin,sstone/bitcoin,JeremyRubin/bitcoin,bitcoinsSG/bitcoin,jtimon/bitcoin,kallewoof/bitcoin,domob1812/namecore,kallewoof/bitcoin,ElementsProject/elements,cdecker/bitcoin,bitcoinsSG/bitcoin,gjhiggins/vcoincore,MeshCollider/bitcoin,jnewbery/bitcoin,Xekyo/bitcoin,fanquake/bitcoin,namecoin/namecoin-core,midnightmagic/bitcoin,MeshCollider/bitcoin,pstratem/bitcoin,CryptArc/bitcoin,jlopp/statoshi,yenliangl/bitcoin,domob1812/bitcoin,yenliangl/bitcoin,namecoin/namecore,fujicoin/fujicoin,wellenreiter01/Feathercoin,fanquake/bitcoin,alecalve/bitcoin,practicalswift/bitcoin,tjps/bitcoin,dscotese/bitcoin,litecoin-project/litecoin,particl/particl-core,bitcoinsSG/bitcoin,rnicoll/bitcoin,rnicoll/dogecoin,jtimon/bitcoin,EthanHeilman/bitcoin,fanquake/bitcoin,sstone/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,namecoin/namecoin-core,midnightmagic/bitcoin,rnicoll/dogecoin,GroestlCoin/GroestlCoin,jonasschnelli/bitcoin,AkioNak/bitcoin,tjps/bitcoin,ElementsProject/elements,apoelstra/bitcoin,mruddy/bitcoin,practicalswift/bitcoin,tecnovert/particl-core,ElementsProject/elements,vertcoin/vertcoin,jnewbery/bitcoin,Sjors/bitcoin,fujicoin/fujicoin,anditto/bitcoin,ajtowns/bitcoin,MarcoFalke/bitcoin,GroestlCoin/bitcoin,midnightmagic/bitcoin,jambolo/bitcoin,AkioNak/bitcoin,bitcoinknots/bitcoin,andreaskern/bitcoin,domob1812/bitcoin,fanquake/bitcoin,andreaskern/bitcoin,gjhiggins/vcoincore,DigitalPandacoin/pandacoin,tjps/bitcoin,vertcoin/vertcoin,litecoin-project/litecoin,tecnovert/particl-core,GroestlCoin/GroestlCoin,CryptArc/bitcoin,JeremyRubin/bitcoin,qtumproject/qtum,EthanHeilman/bitcoin,ahmedbodi/vertcoin,EthanHeilman/bitcoin,mitchellcash/bitcoin,kallewoof/bitcoin,jambolo/bitcoin,gjhiggins/vcoincore,peercoin/peercoin,peercoin/peercoin,rnicoll/dogecoin,bitcoin/bitcoin,gjhiggins/vcoincore,sstone/bitcoin,pataquets/namecoin-core,prusnak/bitcoin,alecalve/bitcoin,qtumproject/qtum,GroestlCoin/bitcoin,domob1812/namecore,dscotese/bitcoin,jambolo/bitcoin,rnicoll/bitcoin,namecoin/namecore,achow101/bitcoin,fanquake/bitcoin,cdecker/bitcoin,achow101/bitcoin,Sjors/bitcoin,EthanHeilman/bitcoin,Xekyo/bitcoin,AkioNak/bitcoin,peercoin/peercoin,namecoin/namecoin-core,kallewoof/bitcoin,DigitalPandacoin/pandacoin,lateminer/bitcoin,jamesob/bitcoin