configure: Fix incorrect string comparison operator

In commit c97d6d2cdf97ed we accidentally added code to configure
that uses '==' for string equality testing. This is a bashism --
the portable way to write this is '='.

This fixes the "Unexpected operator error" complaint produced
if the system /bin/sh is dash.

Fixes: c97d6d2cdf97ed
Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1515431442-23795-1-git-send-email-peter.maydell@linaro.org
