Implement filesystem::canonical() without realpath

Implement filesystem::canonical() without realpath

	PR libstdc++/67173
	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check _XOPEN_VERSION
	and PATH_MAX for _GLIBCXX_USE_REALPATH.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* src/filesystem/ops.cc: (canonical) [!_GLIBCXX_USE_REALPATH]: Add
	alternative implementation.
	* testsuite/experimental/filesystem/operations/canonical.cc: New.
	* testsuite/experimental/filesystem/operations/exists.cc: Add more
	tests.
	* testsuite/experimental/filesystem/operations/absolute.cc: Add test
	variables.
	* testsuite/experimental/filesystem/operations/copy.cc: Likewise.
	* testsuite/experimental/filesystem/operations/current_path.cc:
	Likewise.
	* testsuite/experimental/filesystem/operations/file_size.cc: Likewise.
	* testsuite/experimental/filesystem/operations/status.cc: Likewise.
	* testsuite/experimental/filesystem/operations/temp_directory_path.cc:
	Likewise.

From-SVN: r227836