Auto-detect std::string_view support by default #261

Job	Run time
ubuntu cmake	17s
windows (Win32, standard)	1m 15s
ubuntu (g++, standard)	1m 6s
macos cmake	14s
windows (Win32, PUGIXML_WCHAR_MODE)	1m 37s
ubuntu (g++, PUGIXML_WCHAR_MODE)	1m 12s
windows (Win32, PUGIXML_COMPACT)	1m 20s
ubuntu (g++, PUGIXML_COMPACT)	1m 9s
windows (Win32, PUGIXML_NO_EXCEPTIONS)	1m 25s
ubuntu (g++, PUGIXML_NO_EXCEPTIONS)	1m 2s
windows (x64, standard)	2m 7s
ubuntu (clang++, standard)	52s
windows (x64, PUGIXML_WCHAR_MODE)	1m 33s
windows (x64, PUGIXML_COMPACT)	1m 39s
ubuntu (clang++, PUGIXML_WCHAR_MODE)	51s
windows (x64, PUGIXML_NO_EXCEPTIONS)	1m 38s
ubuntu (clang++, PUGIXML_COMPACT)	49s
ubuntu (clang++, PUGIXML_NO_EXCEPTIONS)	50s
macos (clang++, standard)	43s
macos (clang++, PUGIXML_WCHAR_MODE)	1m 2s
macos (clang++, PUGIXML_COMPACT)	44s
macos (clang++, PUGIXML_NO_EXCEPTIONS)	42s
	24m 7s

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