As in r12-6867-ge20486d508afdf we need to define _GNU_SOURCE explicitly
for Cygwin, because configure finds it in libc but it isn't declared
unless we request it.
libstdc++-v3/ChangeLog:
PR libstdc++/107511
* libsupc++/eh_alloc.cc (_GNU_SOURCE): Define.
// This is derived from the C++ ABI for IA-64. Where we diverge
// for cross-architecture compatibility are noted with "@@@".
+#ifndef _GNU_SOURCE
+// Cygwin needs this for secure_getenv
+# define _GNU_SOURCE 1
+#endif
+
#include <exception> // std::exception
#include <new> // std::terminate
#include <cstdlib> // std::malloc, std::free, std::strtoul