Only MinGW gcc defines WIN32, MSVC compiler does not. It's
also defined by qmake (msvc-desktop.conf), but not by qbs ...
Let's just use _WIN32, that's defined everywhere.
Change-Id: I8342a70498be54a965dcf7fae63eaf406aaa3c04
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>