Disallow out-of-tree builds with config.h in the source tree.

A config.h present in the source tree takes precedence over a config.h
in the build tree, which can possibly result in an incorrect build.
patch by matthieu castet, castet.matthieu free fr

Originally committed as revision 17994 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1212,6 +1212,8 @@ else
     source_path="$(cd "$source_path"; pwd)"
     echo "$source_path" | grep -q '[[:blank:]]' &&
         die "Out of tree builds are impossible with whitespace in source path."
+    test -e "$source_path/config.h" &&
+        die "Out of tree builds are impossible with config.h in source dir."
 fi
 
 FFMPEG_CONFIGURATION="$@"