Martin Storsjö authored Nov 03, 2017 and Tristan Matthews committed Nov 03, 2017

_WIN32 is always defined by the compiler automatically when targeting
that platform, while WIN32 only is defined automatically in some
configurations, and e.g. in MSVC only ever is defined in project files
(if at all).

Some other checks in the codebase already check for both WIN32 and
_WIN32; those are left untouched.