To support Microsoft Visual C++, we must not guard Windows-specific code
in MinGW-specific #ifdef guards.

Happily, even 64-bit MSVC defines the WIN32 constant, therefore we can use
that instead.

[JES: fixed commit message, reordered commit, split out unrelated changes]
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>