build: set AC_PREREQ to 2.69
build: set AC_PREREQ to 2.69

We use build macros such as AX_CHECK_LINK_FLAG, that require >=2.64, so
our configure should also require Autoconf >= 2.64. The build would
already blow up if 2.64 wasn't available. i.e:

configure.ac:320: error: Autoconf version 2.64 or higher is required
build-aux/m4/ax_check_link_flag.m4:74: AX_CHECK_LINK_FLAG is expanded from...

For reference, Autoconf 2.69 was released in April of 2012.
https://lists.gnu.org/archive/html/autoconf/2012-04/msg00041.html

See https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Versioning.html
for more info on AC_PREREQ.

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