Fix libvpx detection

webrtc in 58-based Chromium uses "decoder getters for the last
quantizer" in libvpx. This feautre is not available in the current
latest release of libvpx (1.6.1) thus add config test for the corresponding
macro.

Change-Id: Ib60582bc5401ce0c8960b99c4369bb373ea754d8
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>