cogl-util: Add an internal wrapper for the ffs function

The ffs function is defined in C99 so if we want to use it in Cogl we
need to provide a fallback for MSVC. This adds a configure check for
the function and then a fallback using a while loop if it is not
available.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2491

cogl/cogl-util.c

@@ -218,3 +218,24 @@ _cogl_util_one_at_a_time_mix (unsigned int hash)
   return hash;
 }
 
+/* The 'ffs' function is part of C99 so it isn't always available */
+#ifndef HAVE_FFS
+
+int
+_cogl_util_ffs (int num)
+{
+  int i = 1;
+
+  if (num == 0)
+    return 0;
+
+  while ((num & 1) == 0)
+    {
+      num >>= 1;
+      i++;
+    }
+
+  return i;
+}
+
+#endif /* HAVE_FFS */