If <https://gcc.gnu.org/wiki/Visibility> is right, the symbol _WIN32 is
really confusing since it doesn't mean one is building for 32-bit
Microsoft Windows systems, which is the natural interpretation of that
symbol for those who are not already expert in developing for Microsoft
Windows.
Also, the GNU Coding Standards suggest avoiding using the word "win" as
an abbreviation for Microsoft Windows
<https://www.gnu.org/prep/standards/html_node/Trademarks.html>.
So as far as I'm concerned, the fewer occurrences of _WIN32 in Gnulib,
the better. I'd remove them all if I knew it wouldn't break anything.
Certainly we don't need to add occurrences if the existing code already
works.
