meson: only check for missing networking syms on non-Windows; add compat impls

meson: only check for missing networking syms on non-Windows; add compat impls

These are added in the Makefile, but not in meson. They probably won't
work well on systems without them.

CMake adds them, but only on non-Windows. Actually, it only performs
compiler checks for hstrerror, but excludes that check on Windows with
the note that it is "incompatible with the Windows build". This seems to
be misleading -- it is not incompatible, it simply doesn't exist. Still,
the compat version should not be used.

I interpret this cmake logic to mean we shouldn't even be checking for
symbol availability on Windows. In addition to making it simple to add
compat definitions, this also probably shaves off a second or two of
configure time on Windows as no compiler check needs to be performed.

Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>