Fix macro problem with gai_strerror on Windows.
Commit
5579388d was confused about why gai_strerror() didn't work, and
used gai_strerrorA(). It turns out that we had explicitly undefined
Windows' own macro for that somewhere else. Get rid of all that, and
use the system headers' definition of gai_sterror() directly as
intended.
Discussion: https://postgr.es/m/CA+hUKGKErNfhmvb_H0UprEmp4LPzGN06yR2_0tYikjzB-2ECMw@mail.gmail.com