Thanks for the patch. However, since we used the same idiom elsewhere in the code, I thought that a more extensive change was justified. I committed the fix as http://git.savannah.gnu.org/cgit/findutils.git/commit/?id=a917eb988fab87e47455c73731941d6fbfd1cf69. The fix also cleans output files in directories where the Makefile.am file is generated by gnulib-tool, too.
Thanks, James.
