configure: unbreak cross-pkg-config support with contrib

cross-pkg-config tools have a correct default search path. The contrib
path should complement it, not replace it. If the caller intends to
(ab)use the native pkg-config for cross-compilation, the caller is
responsible for selecting an adequate default path, not VLC configure,
just as with every other build tools.
mmdevice: guard against out of range values
wasapi: fix S/PDIF check
wasapi: fix TimeGet()