Make TH_Depends more portable

We now just sleep for 2 seconds, rather than using
    touch --date="now + 3 seconds"
which doesn't work on OS X. OS X does have "touch -A", but that doesn't
work on Linux.

testsuite/tests/th/Makefile

@@ -29,10 +29,8 @@ TH_Depends:
 # Remove the executable, as GHC won't relink if the timestamps seem to
 # be the same
 	$(RM) TH_Depends TH_Depends.exe
+	sleep 2
 	echo "second run" > TH_Depends_external.txt
-# Give TH_Depends_external.txt a future date in case it gets the same
-# timestamp again
-	touch --date="now + 3 seconds" TH_Depends_external.txt
 	'$(TEST_HC)' $(TEST_HC_OPTS) --make -v0 TH_Depends
 	./TH_Depends