Skip tests by returning 77

automake test runner understands this.
diff --git a/src/check-libstdc++.sh b/src/check-libstdc++.sh
index 40e73b0..bbc8ac3 100755
--- a/src/check-libstdc++.sh
+++ b/src/check-libstdc++.sh
@@ -11,7 +11,7 @@
 	:
 else
 	echo "check-libstdc++.sh: 'ldd' not found; skipping test"
-	exit 0
+	exit 77
 fi
 
 so=.libs/libharfbuzz.so
@@ -23,6 +23,7 @@
 	fi
 else
 	echo "check-libstdc++.sh: libharfbuzz.so not found; skipping test"
+	exit 77
 fi
 
 exit $stat