Added prefixKey to stdenv/generic/setup-new-2.sh

Now you can write prefixKey="-prefix " and "-prefix ${out}" will be inserted in
configureFlags (instead of "--prefix ${out}").

svn path=/nixpkgs/trunk/; revision=9351

pkgs/stdenv/generic/setup-new-2.sh

@@ -595,7 +595,7 @@ configureW() {
     fi
 
     if test -z "$dontAddPrefix"; then
-        configureFlags="--prefix=$prefix $configureFlags"
+        configureFlags="${prefixKey:---prefix=}$prefix $configureFlags"
     fi
 
     echo "configure flags: $configureFlags ${configureFlagsArray[@]}"