On Mac OS X, we should look for glibtool, not for libtool

autogen.sh

@@ -18,8 +18,12 @@
 
 if ! libtool --version > /dev/null
 then
-	echo "Libtool is missing, please install it."
+	# Perhaps we are on a Mac
+	if ! glibtool --version > /dev/null
+	then
+	echo "GNU Libtool is missing, please install it."
 	exit 1
+	fi
 fi
 autoreconf -ivf -I m4