Since commit 417c9d72 all configure tests
normally run with -Werror. Some of these tests now fail because they
raised a compiler warning.

This patch fixes support for capabilities.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>