P0616R0 altered the effects of the <numeric> algorithms to use std::move
on the accumulator values (resolving LWG 2055). This implements the
change for C++2a, but retains the previous behaviour for older
standards.

	* include/bits/stl_numeric.h (_GLIBCXX_MOVE_IF_20): Define macro to
	conditionally move, according to __cplusplus value.
	(accumulate, inner_product, partial_sum, adjacent_difference): Use
	_GLIBCXX_MOVE_IF_20.
	* testsuite/26_numerics/accumulate/lwg2055.cc: New test.
	* testsuite/26_numerics/adjacent_difference/lwg2055.cc: New test.
	* testsuite/26_numerics/inner_product/lwg2055.cc: New test.
	* testsuite/26_numerics/partial_sum/lwg2055.cc: New test.

From-SVN: r262477