configure: make sure pkg-config flags are populated in FT test.
configure: make vp9 decoder select the parser.

The parser is needed. FATE breaks if not present.
lavf: Use av_gettime_relative()

Whenever av_gettime() is used to measure relative period of time,
av_gettime_relative() is prefered as it guarantee monotonic time
on supported platforms.

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>