Add fallback implementation for setenv()

Add fallback implementation for setenv()

This fixes the code compilation on Windows with MSVC and Kerberos, as
a missing implementation of setenv() causes a compilation failure of the
GSSAPI code.  This was only reproducible when building the code with
Kerberos, something that buildfarm animal hamerkop has fixed recently.

This issue only happens on 12 and 13, as this code has been introduced
in b0b39f7.  HEAD is already able to compile properly thanks to
7ca37fb0, and this commit is a minimal cherry-pick of it.

Thanks to Tom Lane for the discussion.

Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 12