* include/bits/erase_if.h [__cplusplus > 201703L]
	(__cpp_lib_erase_if): Only define for C++2a.
	* include/std/iterator [__cplusplus >= 201402L && !_GLIBCXX_DEBUG]
	(__cpp_lib_null_iterators): Define.
	* include/std/version [__cplusplus >= 201402L && !_GLIBCXX_DEBUG]
	(__cpp_lib_null_iterators): Define.
	[__cpp_impl_destroying_delete] (__cpp_lib_destroying_delete): Define.

From-SVN: r267937