Replace WIN32 with _WIN32 everywhere

_WIN32 is defined on all Windows platforms by every compiler that
targets Windows. We do not need WIN32 at all.
2 jobs for !14 with windows-fixes in 14 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Test
passed #10667
docker
whitespace

00:00:14

 
  External
passed continuous-integration/appveyor/merge-request