libstdc++: Define _GNU_SOURCE for secure_getenv on Cygwin [PR107511]

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.