This adds a configure check for the GNU extension secure_getenv and then
uses it for looking up TMPDIR and similar variables.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/65018
	* configure.ac: Check for secure_getenv.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* src/filesystem/ops-common.h (get_temp_directory_from_env): New
	helper function to obtain path from the environment.
	* src/c++17/fs_ops.cc (fs::temp_directory_path): Use new helper.
	* src/filesystem/ops.cc (fs::temp_directory_path): Likewise.
	* testsuite/27_io/filesystem/operations/temp_directory_path.cc:
	Print messages if test cannot be run.
	* testsuite/experimental/filesystem/operations/temp_directory_path.cc:
	Likewise. Fix incorrect condition. Use "TMP" to work with
	Windows as well as POSIX.

Tested powerpc64le-linux. Committed to trunk.