Makefile: fix find -printf issue on FreeBSD

Makefile: fix find -printf issue on FreeBSD

Apparently the use of -printf was causing the error: "printf: missing
format character" with FreeBSD 11.1-RELEASE.  Change the helpers lib
count to rely on -print0 and tr, which should be more portable, but
still be undisturbed by unusual paths.

Thanks to Curtis Dunham for reporting the problem and proposing an
alternate solution.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>