Fixed mkdir warning when running iosbuild.sh multiple times

build-scripts/iosbuild.sh

@@ -11,7 +11,9 @@ fi
 SRC_DIR=$(cd `dirname $0`/..; pwd)
 if [ "$PWD" = "$SRC_DIR" ]; then
     PREFIX=$SRC_DIR/ios-build
+    if [ ! -d "$PREFIX" ]; then
         mkdir $PREFIX
+    fi
 else
     PREFIX=$PWD
 fi