build: actually stop configure if Boost isn't available
build: actually stop configure if Boost isn't available

If Boost is not found via AX_BOOST_BASE, we don't actually stop
configuring, only a warning is emitted:
```bash
checking for boostlib >= 1.58.0 (105800)... configure: We could not detect the boost libraries (version MINIMUM_REQUIRED_BOOST or higher). If you have a staged boost library (still not installed) please specify $BOOST_ROOT in your environment and do not give a PATH to --with-boost option.  If you are sure you have boost installed, then check your version number looking in <boost/version.hpp>. See http://randspringer.de/boost for more documentation.
```

Instead we would usually fail when one of the other
AX_BOOST_* macros fails to find a library. These macros are slowly being
removed, and in any case, it makes more sense to fail earlier if Boost
is missing.

If Boost is unavailable, the failure now looks like:
```bash
checking for boostlib >= 1.58.0 (105800)... configure: We could not detect the boost libraries (version 1.58.0 or higher). If you have a staged boost library (still not installed) please specify $BOOST_ROOT in your environment and do not give a PATH to --with-boost option.  If you are sure you have boost installed, then check your version number looking in <boost/version.hpp>. See http://randspringer.de/boost for more documentation.
configure: error: Boost is not available!
```

Note that we now just pass the version into AX_BOOST_BASE, which fixes
it's display in the output (rather than MINIMUM_REQUIRED_BOOST).

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