In general, if I am understanding rightly ffmpeg more don't install its headers in /ffmeg/.

1. So I've added to my mlt.ebuild

export CFLAGS="$CFLAGS -I$DEST_DIR/include/libavcodec/ -I$DEST_DIR/include/libavformat/ -I$DEST_DIR/include/libswscale/"

It works for kdenlive_builder.sh as well.

2. This code is in //kdenlive/cmake/modules/FindLIBFFMPEG.cmake

#
# Find the FFMPEG includes and library
#

FIND_PATH(LIBFFMPEG_INCLUDE_DIR 
  NAMES ffmpeg/avformat.h
  PATHS ${MLT_PATH}/include /usr/local/include /usr/include
  NO_DEFAULT_PATH
)

So current svn-ffmpeg isn't found.

I think this is rather:

#
# Find the FFMPEG includes and library
#

FIND_PATH(LIBFFMPEG_INCLUDE_DIR 
  NAMES libavformat/avformat.h
  PATHS ${MLT_PATH}/include /usr/local/include /usr/include
  NO_DEFAULT_PATH
)

3. in \thrunk\kdenlive/krender.cpp:28:29 error

kdenlive/krender.cpp:28:29: error: ffmpeg/avformat.h: No such file or directory

Because avformat.h is in libavformat/ not in ffmpeg/