Do not use cached FFMPEG_LIBRARIES. Make error messages clearer.

CMakeLists.txt

@@ -140,6 +140,7 @@ set(OPENMW_LIBS_HEADER)
 
 # Sound setup
 set(FFmpeg_FIND_COMPONENTS AVCODEC AVFORMAT AVUTIL SWSCALE SWRESAMPLE AVRESAMPLE)
+unset(FFMPEG_LIBRARIES CACHE)
 find_package(FFmpeg)
 if ( NOT AVCODEC_FOUND OR NOT AVFORMAT_FOUND OR NOT AVUTIL_FOUND OR NOT SWSCALE_FOUND )
     message(FATAL_ERROR "FFmpeg component required, but not found!")
@@ -150,10 +151,15 @@ if( SWRESAMPLE_FOUND )
     add_definitions(-DHAVE_LIBSWRESAMPLE)
     set(SOUND_INPUT_LIBRARY ${FFMPEG_LIBRARIES} ${SWRESAMPLE_LIBRARIES})
 else()
-    if( AVRESAMPLE_FOUND)
+    # restrict the use of libavresample to debian based system only
+    if(EXISTS "/etc/debian_version")
+        if( AVRESAMPLE_FOUND )
             set(SOUND_INPUT_LIBRARY ${FFMPEG_LIBRARIES} ${AVRESAMPLE_LIBRARIES})
         else()
-        message(FATAL_ERROR "FFmpeg component required, but not found!")
+            message(FATAL_ERROR "Libav libavresample required, but not found!")
+        endif()
+    else()
+        message(FATAL_ERROR "FFmpeg libswresample required, but not found!")
     endif()
 endif()