Auto-detect std::string_view support by default #259

Job	Run time
ubuntu cmake	27s
ubuntu (g++, standard)	1m 7s
macos cmake	14s
windows (Win32, standard)	1m 34s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1m 9s
windows (Win32, PUGIXML_WCHAR_MODE)	1m 35s
ubuntu (g++, PUGIXML_COMPACT)	1m 12s
windows (Win32, PUGIXML_COMPACT)	1m 25s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	1m 3s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 34s
ubuntu (clang++, standard)	52s
windows (x64, standard)	2m 3s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	49s
windows (x64, PUGIXML_WCHAR_MODE)	1m 51s
windows (x64, PUGIXML_COMPACT)	1m 28s
ubuntu (clang++, PUGIXML_COMPACT)	56s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 30s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	45s
macos (clang++, standard)	57s
macos (clang++, PUGIXML_WCHAR_MODE)	1m 14s
macos (clang++, PUGIXML_COMPACT)	1m 15s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	49s
	25m 49s

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