build: Tweak CMakeLists.txt pkg-config usage for finding avutil.

* CMakeLists.txt: Prepend CMAKE_PREFIX_PATH with the location of
libavutil.pc from daemon's contrib before trying to find libavutil
using pkg-config, and remove it afterwards.  Further, there is no need
to link the application executable against libavutil; the application
only needs its headers (libclient, however, does already link against
libavutil).

Change-Id: Iffa5af43bfd1bb62bc24cc94dce292f87e9ecb2a

CMakeLists.txt

@@ -361,7 +361,12 @@ elseif (NOT APPLE)
   endif()
 
   pkg_check_modules(XCB xcb)
+
+  list(PREPEND CMAKE_PREFIX_PATH
+    ${LIBJAMI_CONTRIB_DIR}/native/ffmpeg/libavutil)
   pkg_check_modules(LIBAVUTIL libavutil>=55.75.100)
+  list(REMOVE_ITEM CMAKE_PREFIX_PATH
+    ${LIBJAMI_CONTRIB_DIR}/native/ffmpeg/libavutil)
 
   include_directories(
     ${LIBCLIENT_SRC_DIR}
@@ -557,8 +562,7 @@ elseif (NOT APPLE)
     ${LIBGDKPIXBUF_LIBRARIES}
     ${GLIB_LIBRARIES}
     ${GIO_LIBRARIES}
-    ${XCB_LIBRARIES}
-    ${LIBAVUTIL_LIBRARIES})
+    ${XCB_LIBRARIES})
 
   # Installation rules
   install(