build: drop support for Libav 9

I have no tolerance for this garbage anymore. There are tons of issues
with it (see e.g. previous commit), and there is no reason to use it
either. Use Libav git, or Libav 10 when it's released.

This also drops support for earlier FFmpeg release, which have exactly
the same issues as Libav 9. FFmpeg 2.1.4 is still supported, because
it's the latest release, and is reasonably recent. (Although this will
probably also be dropped as soon as FFmpeg 2.2 is released.)

Assumed version table (lots of nonsensical numbers):

        FFmpeg 2.1.4    FFmpeg (n2.2-rc2)   Libav (v10_beta2)

lavu    52.48.101       52.66.100           53.3.0
lavc    55.39.101       55.52.102           55.34.1
lavf    55.19.104       55.33.100           55.12.0
lsws    2.5.101         2.5.101             2.1.2
lavi    3.90.100        4.2.100             4.2.0
lswr    0.17.104        0.18.100            -
lavr    1.1.0           1.2.0               1.1.0

libpostproc and libavdevice are not interesting.

Following this commit, code needed just to support old Libav versions
will start to be removed.