Bump libavutil major version to account for the LLS API/ABI changes.

Commit 41578f70cf8aec8e7565fba1ca7e07f3dc46c3d2 changed the LLS API, which was
called from libavcodec. Thus using an old libavcodec with a new libavutil will
break.

All scheduled API changes are deferred to the next bump.
Bump major of libavfilter for reference counted buffer API changes.

Applications that have been linked against an older release of Libav and pick up
updated libraries experience segmentation faults because they pick up the new
libavfilter, which assumes AVFrames have been allocated by libavutil and thus
contain new reference-counting related fields. This will break for AVFrames that
have been allocated by old libavcodec.

All scheduled API changes are deferred to the next bump.
build: Define __printf__ to __gnu_printf__ on MinGW*/gcc

This is necessary to avoid GCC assuming MS style printf arguments.
avutil: Remove deprecated intfloat_readwrite code

It was deprecated over two years ago.