doc: Clang 8 or later is required with FORCE_USE_SYSTEM_CLANG
doc: Clang 8 or later is required with FORCE_USE_SYSTEM_CLANG

The usage of pragmas within the macOS SDK requires LLVM Clang 8. This is
the version as our prebuilt Clang, however the minimum is worth noting here
as they may diverge and/or expert users might expect they could use an
earlier version.

If you compile using Clang 7 you'll see output like:
```bash
In file included from kernel/qcore_mac_objc.mm:44:
In file included from /bitcoin/depends/SDKs/Xcode-11.3.1-11C505-extracted-SDK-with-libcxx-headers/System/Library/Frameworks/AppKit.framework/Headers/NSText.h:9:
In file included from /bitcoin/depends/SDKs/Xcode-11.3.1-11C505-extracted-SDK-with-libcxx-headers/System/Library/Frameworks/AppKit.framework/Headers/NSView.h:19:
In file included from /bitcoin/depends/SDKs/Xcode-11.3.1-11C505-extracted-SDK-with-libcxx-headers/System/Library/Frameworks/AppKit.framework/Headers/NSResponder.h:10:
/bitcoin/depends/SDKs/Xcode-11.3.1-11C505-extracted-SDK-with-libcxx-headers/System/Library/Frameworks/AppKit.framework/Headers/NSEvent.h:19:1: error:
      expected 'push' or 'pop' after '#pragma clang attribute'
/bitcoin/depends/SDKs/Xcode-11.3.1-11C505-extracted-SDK-with-libcxx-headers/usr/include/os/availability.h:104:273: note: expanded from macro
      'API_UNAVAILABLE_BEGIN'
  ...__API_UNAVAILABLE_BEGIN5, __API_UNAVAILABLE_BEGIN4, __API_UNAVAILABLE_BEGIN3, __API_UNAVAILABLE_BEGIN2, __API_UNAVAILABLE_BEGIN1, 0)(__VA_A...
                                                                                                             ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
```

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