[Bug bootstrap/67546] bootstrap broken on x86_64-w64-mingw32, error: '::unsetenv' has not been declared in gcc.c

[ktietz at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67546

--- Comment #6 from Kai Tietz <ktietz at gcc dot gnu.org> ---
Initially in PR/67363 just the missing strndup due failed libiberty-linking was
noticed.  Later comments are duplicate of this.
So feel free to mark one of these bugs as duplicate, if you prefer.