Fix libvpx check

We no longer depend on a custom configuration of libvpx and shouldn't
check for non-standard headers.

Also fixes bitrot in the config.test.

Task-number: QTBUG-50554
Change-Id: I7b625e735536f7a79e41d036b7b8dc4d79f2262b
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>