[Bug libstdc++/107511] [13 Regression] gcc-13-20221030 failure to build on Cygwin due to lack of secure_getenv

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107511

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect adding:
#ifndef _GNU_SOURCE
// Cygwin needs this for secure_getenv
# define _GNU_SOURCE 1
#endif

at the beginging of eh_alloc.cc fixes the issue; just like what was done for
src/c++17/fs_ops.cc, src/filesystem/dir.cc, etc.