Honor ./configure --cc= option

The cc variable is assigned before parsing command-line options.  This
effectively ignores the ./configure --cc= option.  Move the cc variable
assigned below option parsing.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
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>