Add mapping for SMPTE 302m audio
libav: update to v9.1
Fix 'make dist'

This time for real hopefully. Make sure to remove any
libav-dist directories that may still be around from
previous make dist attempts first.
Add uninstalled pkg-config file so other modules can find libav elements for unit tests

e.g. gst-plugins-bad has some unit tests that draw on av* elements.
swscale: Use 1.0-style video/x-raw caps
libav: update to v9.3
Automatic update of common submodule

From 2de221c to 04c7a1e
configure: Add AG_GST_ARG_GCOV to make automake happy again

+ running automake -a -c -Wno-portability...
common/check.mak:107: error: GST_GCOV_ENABLED does not appear in AM_CONDITIONAL
tests/check/Makefile.am:1:   'common/check.mak' included from here
configure: Set the assembler used for libav to $CC

libav assumes that it's not just an assembler but something that
can also handle CPPFLAGS and other things.

https://bugzilla.gnome.org/show_bug.cgi?id=694416
Fuck libav. Long live ffmpeg!