Backpatch: Fix warnings about declaration of environ on MinGW

Backpatch: Fix warnings about declaration of environ on MinGW

Backpatch commit 7bc9a8bdd2d to 13-17. The motivation for backpatching is that
we want to update CI to Debian Trixie. Trixie contains a newer mingw
installation, which would trigger the warning addressed by 7bc9a8bdd2d. The
risk of backpatching seems fairly low, given that it did not cause issues in
the branches the commit is already present.

While CI is not present in 13-14, it seems better to be consistent across
branches.

Author: Thomas Munro <tmunro@postgresql.org>
Discussion: https://postgr.es/m/o5yadhhmyjo53svzwvaocww6zkrp63i4f32cw3treuh46pxtza@hyqio5b2tkt6
Backpatch-through: 13