This removes some clutter in compilation logging, and allows some
easier tweaking per compilation unit/CFLAGS overriding.

Note that we can't move those define in os-win32.h, since they must be
set before the first system headers are included.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181122110039.15972-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>