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

diff --git a/configure b/configure
index c8648d4..d553b1a 100755
--- a/configure
+++ b/configure
@@ -16148,6 +16148,7 @@
 
 
 
+
 for ac_func in \
 	_lwp_create \
 	_lwp_self \
@@ -16161,6 +16162,7 @@
 	chsize \
 	clone \
 	epoll_create \
+	ffs \
 	finite \
 	fork \
 	fpclass \