libstdc++: Define _GNU_SOURCE for secure_getenv on Cygwin [PR104217]

libstdc++: Define _GNU_SOURCE for secure_getenv on Cygwin [PR104217]

For GNU/Linux G++ defines _GNU_SOURCE automatically, but not for Cygwin.
This means secure_getenv is not declared by Cygwin's <stdlib.h>, even
though autoconf detected it is present in the library. Define it in the
source files that want to use secure_getenv.

libstdc++-v3/ChangeLog:

	PR libstdc++/104217
	* src/c++17/fs_ops.cc (_GNU_SOURCE): Define.
	* src/filesystem/dir.cc (_GNU_SOURCE): Define.
	* src/filesystem/ops.cc (_GNU_SOURCE): Define.