alsa-audio-dec: explicitly cast the delay to a signed int64

Otherwise the expression will be evaluated as unsigned, which will break
when the result should be negative.
CC:libav-stable@libav.org
avconv: initialize hwaccel_pix_fmt

Fixes an assertion failure in vdpau_get_buffer() when -hwaccel auto is
used, but hwaccel init fails.
configure: Add detected C library and host compiler to informative output
configure: Detect msvcrt libc with a CPP check instead of a link check

Simplifies host/target libc detection splitting.
x86: Initialize mmxext after amd3dnow optimizations

The mmxext optimizations should be at least equally fast if available and
amd3dnow optimizations are being deprecated. Thus the former should
override the latter, not the other way around.
mpeg4videodec: move use_intra_dc_vlc from MpegEncContext to Mpeg4DecContext
mpegvideo: remove an unused variable
mpeg12dec: move first_slice from MpegEncContext to Mpeg1Context
mpeg4videodec: move intra_dc_threshold from MpegEncContext to Mpeg4DecContext
mpeg4videodec: move showed_packed_warning from MpegEncContext to Mpeg4DecContext
mpegvideo: don't copy input_picture_number in update_thread_context()

It is encoding-only, frame threading is not used for encoding.
mpegvideo: move setting encoding-only vars from common_defaults() to encode_defaults()
mpegvideo: move encoding-only initialization from common_init() to encode_init()
mpegvideo: do not update last_non_b_pict_type in update_thread_context()

It is used for encoding only.
mpegvideo: split the encoding-only parts of ff_MPV_frame_end() into a separate function
mpegvideo: remove commented out cruft from ff_MPV_frame_end()
mpegvideo: move encode-only parts of common_end() to encode_end()
mpegvideo: make frame_size_alloc() static.

It is not called from outside of mpegvideo.c anymore.
mpegvideo: make ff_release_unused_pictures() static

It is only called from one place in mpegvideo.c now. Also remove the
remove_current parameter, which is always 1.
vc1dec: move setting repeat_pict after frame_start() has been called.

This will allow removing the hacks where each decoder sets
current_picture_ptr on its own.