configure: use -Wwombat to test whether gcc recognizes -Wno-wombat

gcc will silently accept unrecognized -Wno-wombat warning suppression
options (it only mentions them if it has to print a compiler warning
for some other reason). Since we already run a check for whether gcc
recognizes the warning options we use, we can easily make this use
the positive sense of the option when checking for support for the
suppression option. This doesn't have any effect except that it avoids
gcc emitting extra messages about unrecognized command line options
when it is printing other warning messages.

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