configure, avcodec/Makefile: Add new mpeg4audio CONFIG_EXTRA group

This group is mainly for the users of *_mpeg4audio_get_config2();
it is not for those who only use avpriv_mpeg4audio_sample_rates.
This is in preparation for splitting the latter into a file of its own;
if there were no CONFIG_EXTRA group for *_mpeg4audio_get_config2()
users, one would have to add a dependency to the new file for all
these users on top of the existing dependency on mpeg4audio.o.

Adding a new CONFIG_EXTRA group only takes effect after a reconfigure;
so in order to force a reconfigure some unnecessary headers from
libavdevice/alldevices.c have been removed.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

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