cmake: properly find and link ffmpeg's libavutil on GNU/Linux

Change-Id: I41fbc747e0868b70edb23232bb8bfb10ac0f225d

CMakeLists.txt

@@ -197,16 +197,13 @@ set(COMMON_HEADERS
     ${SRC_DIR}/videoprovider.h
 )
 
-# For avframe dependency on Windows/macOS.
-if(NOT DEFINED LIBAV_INCLUDE_PATH)
-   set(LIBJAMI_CONTRIB_DIR "${PROJECT_SOURCE_DIR}/../daemon/contrib/")
-   if(WIN32)
-      set(LIBAV_INCLUDE_PATH ${LIBJAMI_CONTRIB_DIR}/build/ffmpeg/Build/win32/x64/include/)
-   else()
-      set(LIBAV_INCLUDE_PATH ${LIBJAMI_CONTRIB_DIR}/native/ffmpeg)
-   endif()
-endif()
-include_directories(${LIBAV_INCLUDE_PATH})
+# For libavutil/avframe.
+set(LIBJAMI_CONTRIB_DIR "${PROJECT_SOURCE_DIR}/../daemon/contrib")
+find_path(AVUTIL_INCLUDE_DIR libavutil/avutil.h
+   PATHS
+   ${LIBJAMI_CONTRIB_DIR}/native/ffmpeg
+   ${LIBJAMI_CONTRIB_DIR}/build/ffmpeg/Build/win32/x64/include)
+include_directories(${AVUTIL_INCLUDE_DIR})
 
 if(MSVC)
     set(WINDOWS_SYS_LIBS