libstdc++: Fix filebuf::native_handle() for Windows

libstdc++: Fix filebuf::native_handle() for Windows

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_CHECK_FILEBUF_NATIVE_HANDLES): Add
	missing header to configure test. Check correct variable.
	* config/io/basic_file_stdio.cc (__basic_file<char>::native_handle):
	Fix typo.
	* configure: Regenerate.
	* testsuite/27_io/basic_filebuf/native_handle/char/1.cc: Do not
	call CloseHandle on the native handle.
	* testsuite/27_io/basic_filebuf/native_handle/wchar_t/1.cc:
	Likewise.