build: Add --with-append-cxxflags option
build: Add --with-append-cxxflags option

Github-Pull: #22159
Rebased-From: fa14c6818f4094669584a110a517fa1347f1f36e

bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin