Don't test for attribute visibility, because it doesn't support it, but it...

Don't test for attribute visibility, because it doesn't support it, but it will pass the test and fill the warning output with messages.
Profit to s/enable-pp/enable-postproc in the comments.

configure.ac

@@ -1062,6 +1062,10 @@ if test "${ac_cv_c_omit_frame_pointer}" != "no"; then
 fi
 
 dnl Check for -fvisibility=hidden
+dnl Don't test on mingw32, because it is going to work with just a warning
+dnl even if it doesn't support it
+if test "${SYS}" != "mingw32"
+then
 AC_CACHE_CHECK([if \$CC accepts -fvisibility=hidden],
     [ac_cv_c_visibility_hidden],
     [CFLAGS="${CFLAGS_save} -fvisibility=hidden"
@@ -1069,6 +1073,7 @@ AC_CACHE_CHECK([if \$CC accepts -fvisibility=hidden],
 if test "${ac_cv_c_visibility_hidden}" != "no"; then
     VLC_ADD_CFLAGS([libvlc plugin],[-fvisibility=hidden])
 fi
+fi
 
 AM_CONDITIONAL(HAVE_COMPILER_EXPORT, [test "$SYS" = "mingw32" -o "${ac_cv_c_visibility_hidden}" != "no"])
 
@@ -3004,9 +3009,9 @@ dnl Those options have to be here because the .pc can be bogus for ffmpeg previo
         VLC_ADD_LIBS([ffmpeg],[-L${real_ffmpeg_tree}/libpostproc ${real_ffmpeg_tree}/libpostproc/libpostproc.a])
         VLC_ADD_CPPFLAGS([ffmpeg stream_out_switcher],[-I${real_ffmpeg_tree}])
      else
-       dnl  The given libavcodec wasn't built with --enable-pp
+       dnl  The given libavcodec wasn't built with --enable-postproc
        AC_MSG_RESULT(no)
-       AC_MSG_ERROR([cannot find libpostproc.a in ${real_ffmpeg_tree}/[libavcodec/]libpostproc/. Make sure you configured ffmpeg with --enable-pp])
+       AC_MSG_ERROR([cannot find libpostproc.a in ${real_ffmpeg_tree}/[libavcodec/]libpostproc/. Make sure you configured ffmpeg with --enable-postproc])
      fi
    fi
    dnl  Use a custom libffmpeg
@@ -3118,7 +3123,7 @@ dnl Trying with pkg-config
 
       AC_CHECK_LIB(postproc, pp_postprocess, [
         VLC_ADD_LIBS([ffmpeg],[-lpostproc])],
-      AC_MSG_ERROR([Could not find libpostproc inside FFmpeg. You should configure FFmpeg with --enable-gpl --enable-pp.]),
+      AC_MSG_ERROR([Could not find libpostproc inside FFmpeg. You should configure FFmpeg with --enable-gpl --enable-postproc.]),
         [$LDAVUTIL])
 
       AC_CHECK_LIB(avformat, av_open_input_stream, [
@@ -3156,9 +3161,9 @@ then
       AC_MSG_ERROR([cannot find ${real_ffmpeg_tree}/libavcodec/libavcodecaltivec.a, make sure you compiled libavcodecaltivec in ${with_ffmpeg_tree}])
     fi
     if ! fgrep -s "pp_get_context" "${real_ffmpeg_tree}/libavcodec/libavcodecaltivec.a"; then
-      dnl  The given libavcodecaltivec wasn't built with --enable-pp
+      dnl  The given libavcodecaltivec wasn't built with --enable-postproc
       AC_MSG_RESULT(no)
-      AC_MSG_ERROR([${real_ffmpeg_tree}/libavcodec/libavcodecaltivec.a was not compiled with postprocessing support, make sure you configured ffmpeg with --enable-pp])
+      AC_MSG_ERROR([${real_ffmpeg_tree}/libavcodec/libavcodecaltivec.a was not compiled with postprocessing support, make sure you configured ffmpeg with --enable-postproc])
     fi
     dnl  Use a custom libffmpeg
     AC_MSG_RESULT(${real_ffmpeg_tree}/libavcodec/libavcodecaltivec.a)