fix broken url

Seirra · murraycu · commit 94c1f9a4c9d6 · 2018-06-11T12:41:56.000+02:00
current url in AC_INIT is broken, I imagine the github webpage serves as a good stand in replacement?
diff --git a/configure.ac b/configure.ac
@@ -17,7 +17,7 @@
 
 AC_INIT([libsigc++], [2.99.11],
         [http://bugzilla.gnome.org/enter_bug.cgi?product=libsigc%2B%2B],
-        [libsigc++], [http://libsigc.sourceforge.net/])
+        [libsigc++], [https://libsigcplusplus.github.io/libsigcplusplus/])
 AC_PREREQ([2.59])
 
 AC_CONFIG_SRCDIR([sigc++/sigc++.h])
