Fix checking for existence of compress binary.

whereis does not return an error if it doesn't find anything, so
look at the whereis output instead.  If there is any, then if found
the executable on the path.

Makefile

@@ -39,7 +39,7 @@ test: pigz
 	(printf "w" | gzip ; printf "x") | ./pigz -cdf | wc -c | test `cat` -eq 2
 	(printf "w" | gzip ; printf "xy") | ./pigz -cdf | wc -c | test `cat` -eq 3
 	(printf "w" | gzip ; printf "xyz") | ./pigz -cdf | wc -c | test `cat` -eq 4
-	-@if whereis compress > /dev/null; then \
+	-@if test "`whereis compress`" != ""; then \
 	  echo 'compress -f < pigz.c | ./unpigz | cmp - pigz.c' ;\
 	  compress -f < pigz.c | ./unpigz | cmp - pigz.c ;\
 	fi