fix issues with latest versions of libxml2 #516

Job	Run time
build-win64 (Debug, windows-2019)	7m 8s
build-linux (Debug, ubuntu-latest)	3m 16s
build-win64 (Debug, windows-latest)	5m 54s
build-linux (Debug, ubuntu-20.04)	2m 57s
build-win64 (Release, windows-2019)	9m 50s
build-linux (Release, ubuntu-latest)	1m 6s
build-win64 (Release, windows-latest)	5m 54s
build-linux (Release, ubuntu-20.04)	3m 33s
test-linux (Debug, ubuntu-latest)	4s
test-linux (Debug, ubuntu-20.04)	4s
test-linux (Release, ubuntu-latest)	3s
test-linux (Release, ubuntu-20.04)	9s
test-win64 (Debug, windows-2019)	12s
test-win64 (Debug, windows-latest)	10s
test-win64 (Release, windows-2019)	10s
test-win64 (Release, windows-latest)	10s
deploy-win64	36s
	41m 16s

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