Detect Windows operating system in ./configure

We should not rely on `uname -s` if we ever want cross-compiling to
work, so detect _WIN32.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/configure b/configure
index 7b1d79a..63aa02d 100755
--- a/configure
+++ b/configure
@@ -184,6 +184,8 @@
 elif check_define __sun__ ; then
   targetos='SunOS'
   CFLAGS="$CFLAGS -D_REENTRANT"
+elif check_define _WIN32 ; then
+  targetos='CYGWIN'
 else
   targetos=`uname -s`
 fi