Fix copy_headers.sh

copy_headers.sh

@@ -45,8 +45,8 @@ for src_file in $(echo | $CLANG -M -xc++ -std=c++1z -Wall -Werror -msse4 -mcx16
     sed -r -e 's/^-\.o://');
 do
     dst_file=$src_file;
-    dst_file=$(echo $dst_file | sed -r -e 's/build\///')    # for simplicity reasons, will put generated headers near the rest.
     [ -n $DESTDIR ] && dst_file=$(echo $dst_file | sed -r -e "s!^$DESTDIR!!")
+    dst_file=$(echo $dst_file | sed -r -e 's/build\///')    # for simplicity reasons, will put generated headers near the rest.
     mkdir -p "$DST/$(echo $dst_file | sed -r -e 's/\/[^/]*$/\//')";
     cp "$src_file" "$DST/$dst_file";
 done