libstdc++: testsuite: conditionalize symlink tests

libstdc++: testsuite: conditionalize symlink tests

Several filesystem tests expect to be able to create symlinks even
when !defined (_GLIBCXX_HAVE_SYMLINK), and fail predictably, reducing
the amount of testing of other filesystem features.

They are already skipped for mingw targets.  I've extended the
skipping to other targets in which _GLIBCXX_HAVE_SYMLINK is undefined,
through a new NO_SYMLINKS macro in testsuite_fs.h that guards
skippable portions of tests, and dg-require-target-fs-symlinks for
tests that would be reduced to nothing.

for  libstdc++-v3/ChangeLog

	* testsuite/util/testsuite_fs.h (NO_SYMLINKS): Define on
	mingw and when create_symlink is a dummy.
	* testsuite/27_io/filesystem/operations/symlink_status.cc:
	Drop mingw xfail.
	(test01, test02): Don't create symlinks when NO_SYMLINKS is
	defined.
	* testsuite/27_io/filesystem/operations/canonical.cc (test03):
	Likewise.
	* testsuite/27_io/filesystem/operations/copy.cc (test02):
	Likewise.
	* testsuite/27_io/filesystem/operations/create_directories.cc
	(test04): Likewise.
	* testsuite/27_io/filesystem/operations/create_directory.cc
	(test01): Likewise.
	* testsuite/27_io/filesystem/operations/permissions.cc
	(test03, test04): Likewise.
	* testsuite/27_io/filesystem/operations/remove.cc (test01):
	Likewise.
	* testsuite/27_io/filesystem/operations/remove_all.cc (test01):
	Likewise.
	* testsuite/27_io/filesystem/operations/rename.cc
	(test_symlinks): Likewise.
	* testsuite/27_io/filesystem/operations/weakly_canonical.cc
	(test01): Likewise.
	* testsuite/experimental/filesystem/iterators/recursive_directory_iterator.cc
	(test06): Likewise.
	* testsuite/experimental/filesystem/operations/copy.cc
	(test01): Likewise.
	* testsuite/experimental/filesystem/operations/create_directories.cc
	(test04): Likewise.
	* testsuite/experimental/filesystem/operations/create_directory.cc
	(test01): Likewise.
	* testsuite/experimental/filesystem/operations/permissions.cc
	(test03, test04): Likewise.
	* testsuite/experimental/filesystem/operations/remove.cc
	(test01): Likewise.
	* testsuite/experimental/filesystem/operations/remove_all.cc
	(test01): Likewise.
	* testsuite/experimental/filesystem/operations/rename.cc
	(test01): Likewise.
	* testsuite/lib/libstdc++.exp
	(v3_check_preprocessor_condition): Add optional inc parameter.
	Add it to the test program after include bits/c++config.h.
	(check_v3_target_fs_symlinks): New.
	* testsuite/lib/dg-options.exp
	(dg-require-target-fs-symlinks): New.
	* testsuite/27_io/filesystem/operations/read_symlink.cc:
	Replace mingw xfail with require target-fs-symlinks.
	* testsuite/experimental/filesystem/operations/read_symlink.cc:
	Likewise.