libstdc++: Define __cpp_lib_three_way_comparison conditionally

libstdc++: Define __cpp_lib_three_way_comparison conditionally

The contents of the <compare> header are not complete unless concepts
are supported, so the feature test macro should depend on the macro for
concepts.

As a result, the std::lexicographical_compare_three_way function will
not be defined unless concepts are supported, so there is no need to
check __cpp_lib_concepts before using concepts in those functions.

	* include/bits/stl_algobase.h (__is_byte_iter, __min_cmp)
	(lexicographical_compare_three_way): Do not depend on
	__cpp_lib_concepts.
	* include/std/version (__cpp_lib_three_way_comparison): Only define
	when __cpp_lib_concepts is defined.
	* libsupc++/compare (__cpp_lib_three_way_comparison): Likewise.

From-SVN: r279896