Add ffs() to porting library, needed by MinGW.

diff --git a/configure.ac b/configure.ac
index 5e95810..7028730 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1076,6 +1076,7 @@
 	chsize \
 	clone \
 	epoll_create \
+	ffs \
 	finite \
 	fork \
 	fpclass \