FFMPEG_INCLUDE_DIRS must be a list not a string.

Changes CXX_INCLUDES:
- ... -I"/usr/include/ffmpeg /usr/include/ffmpeg" ...
+ ... -I/usr/include/ffmpeg ...

Fixes:
src/backends/decoder.h:29:32: fatal error: libavcodec/avcodec.h: No such file or directory

CMakeLists.txt

@@ -296,7 +296,7 @@ IF(ENABLE_LIBAVCODEC)
   INCLUDE(CheckFunctionExists REQUIRED)
   INCLUDE(CheckCSourceCompiles)
   SET(FFMPEG_FLAGS "${LIBAVCODEC_CFLAGS} ${LIBAVRESAMPLE_FLAGS}")
-  SET(FFMPEG_INCLUDE_DIRS "${FFMPEG_INCLUDE_DIRS} ${LIBAVRESAMPLE_INCLUDE_DIRS}")
+  SET(FFMPEG_INCLUDE_DIRS "${FFMPEG_INCLUDE_DIRS};${LIBAVRESAMPLE_INCLUDE_DIRS}")
   SET(FFMPEG_LIBRARIES "${FFMPEG_LIBRARIES};${LIBAVRESAMPLE_LIBRARIES}")
   SET(CMAKE_REQUIRED_FLAGS ${FFMPEG_FLAGS})
   SET(CMAKE_REQUIRED_INCLUDES ${FFMPEG_INCLUDE_DIRS})