--- a/tools-for-build/whitespacely-canonical-filenames
+++ b/tools-for-build/whitespacely-canonical-filenames
@@ -14,6 +14,14 @@
 #   .sh: if ./make.sh is altered, Bad Things happen
 #   ?
 
+stamp_file=whitespace-stamp
+
+if [ -e $stamp_file ]; then
+    find_opt="-newer $stamp_file"
+else
+    find_opt=""
+fi
+
 for source_extension in $source_extensions; do 
     find . \( \
             -name _darcs -o \
@@ -21,5 +29,7 @@
             -name CVS -o \
             -name .hg -o \
             -name .svn \) -type d -prune -o \
-           -name '*'$source_extension -print
+           $find_opt -name '*'$source_extension -print
 done
+
+touch $stamp_file