make depend: Check that find returned a non-empty string rather than an empty

The logic to find out of there are any .d files newer than Makefile is
sound.  Checking the result was less so.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Configurations/unix-Makefile.tmpl

@@ -261,7 +261,7 @@ clean: libclean
 # concatenate only if that is true.
 depend:
 	@: {- output_off() if $disabled{makedepend}; "" -}
-	@if [ -z "`find $(DEPS) -newer Makefile 2>/dev/null; exit 0`" ]; then \
+	@if [ -n "`find $(DEPS) -newer Makefile 2>/dev/null; exit 0`" ]; then \
 	  ( sed -e '/^# DO NOT DELETE THIS LINE.*/,$$d' < Makefile; \
 	    echo '# DO NOT DELETE THIS LINE -- make depend depends on it.'; \
 	    echo; \