Compilation error with newer ffmpeg version

[ismangil]

Newer ffmpeg version changed variable names, resulting in build error:

In file included from ../src/pjmedia/ffmpeg_util.c:26:
../src/pjmedia/ffmpeg_util.h:50:12: warning: declaration of 'enum CodecID' will
      not be visible outside of this function [-Wvisibility]
                                         enum CodecID *codec_id);
                                              ^
../src/pjmedia/ffmpeg_util.h:52:47: warning: declaration of 'enum CodecID' will
      not be visible outside of this function [-Wvisibility]
pj_status_t CodecID_to_pjmedia_format_id(enum CodecID codec_id,
                                              ^
../src/pjmedia/ffmpeg_util.c:65:18: error: field has incomplete type
      'enum CodecID'
    enum CodecID        codec_id;
                        ^

Reported in ​http://lists.pjsip.org/pipermail/pjsip_lists.pjsip.org/2014-March/017286.html

[ismangil]

Patch to check ffmpeg version

[ismangil]

Contributed patch from original reporter attached

[ismangil]

In 4801:

Re #1750 fix for detecting definition of CodecID in newer ffmpeg

[nanang]

In 4808:

Re #1750:

• Replaced "enum CodecID" with "unsigned" (previous fix raised compile error for some older ffmpeg versions).
• Handled PIX_FMT_GBR24P hassle.