Various patches related to FFMpeg

[riza]

This ticket will contain various patches related to FFMpeg.

For example, newer FFMpeg might deprecate API and introduce a new one.

[riza]

In 5301:

Re #1920: In the development branch of FFmpeg 0.8 (in July 2010), a new library
was created AVCore. Just a half year later (February 2011), AVCore was
merged into AVUtil (52.38.0).

Furthermore, nothing in pjproject uses libavcore. Therefore, it is save
to remove all references to that library.

Thanks to Alexander Traud for the patch.

[riza]

In 5302:

Re #1920: Since FFmpeg version 0.7 (June 2011), the struct AVFormatParameters,
the function av_open_input_stream, and function av_close_input_stream
are deprecated. With FFmpeg 0.11, those three symbols were removed.

Thanks to Alexander Traud for the patch.

[riza]

In 5303:

Re #1920: Fix typo on configuration script when trying to detect FFMpeg
package (libavdevice).

Thanks to Alexander Traud for the patch.

[riza]

In 5304:

Re #1920: Check for libavformat instead of libavcodec when setting
PJMEDIA_HAS_FFMPEG_CODEC.

ffmpeg_vid_codecs.c has a compile-time guard of PJMEDIA_HAS_FFMPEG_VID_CODEC
which is PJMEDIA_HAS_FFMPEG_CODEC which is PJMEDIA_HAS_LIBAVCODEC.
However, the code requires libavutil, libavcodec, and libavformat.
In Ubuntu (and Debian), the package libavformat-dev contains
libavcodec-dev and libavutil-dev.
Not the other way around. Therefore, the current compile-time guard is
not sufficient and should check PJMEDIA_HAS_LIBAVFORMAT instead.

Thanks to Alexander Traud for the patch.

[riza]

In 5305:

Re #1920: Update how keyframe is detected on FFMpeg codec, since
AVCodecContext.coded_frame is deprecated.

[riza]

In 5306:

Re #1920: Avoid linking error when video is not enabled.