[Bug libstdc++/65018] Use secure_getenv when available

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65018

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> From Jakub on IRC: "note that at some point, there used to be
> __secure_getenv instead. perhaps look at libgfortran configury what it does
> for this"

secure_getenv was added to glibc in 2.17, before then only the internal
__secure_getenv was provided. But I don't think we need to care too much about
pre-2.17 so I'm not going to bother checking for __secure_getenv.

The profile mode has been removed from libstdc++, so we don't need to care
about those variables now. The others in comment 0 are not a problem. Only the
queries for TMPDIR etc need to use secure_getenv. I am testing a patch for
that.