fixed compile errors on OSX and other systems where ffmpeg is detected manually, not via pkg-config

cmake/OpenCVFindLibsVideo.cmake

@@ -229,6 +229,18 @@ if(WITH_FFMPEG)
         find_library(FFMPEG_UTIL_LIB "avutil" HINTS "${FFMPEG_LIB_DIR}")
         find_library(FFMPEG_SWSCALE_LIB "swscale" HINTS "${FFMPEG_LIB_DIR}")
         find_library(FFMPEG_RESAMPLE_LIB "avresample" HINTS "${FFMPEG_LIB_DIR}")
+        if(FFMPEG_CODEC_LIB)
+          set(HAVE_FFMPEG_CODEC 1)
+        endif()
+        if(FFMPEG_FORMAT_LIB)
+          set(HAVE_FFMPEG_FORMAT 1)
+        endif()
+        if(FFMPEG_UTIL_LIB)
+          set(HAVE_FFMPEG_UTIL 1)
+        endif()
+        if(FFMPEG_SWSCALE_LIB)
+          set(HAVE_FFMPEG_SWSCALE 1)
+        endif()
         if(FFMPEG_CODEC_LIB AND FFMPEG_FORMAT_LIB AND
            FFMPEG_UTIL_LIB AND FFMPEG_SWSCALE_LIB)
           set(ALIASOF_libavcodec_VERSION "Unknown")