configure: check for $cxx before use

I broke this when adding checks for clang++.

Reported-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1447345789-840-1-git-send-email-jsnow@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
