Introduce a simple timer API....

One can use add_timer or schedule_timer to add a timer. After
the timeout time has been reached the callback will be called.
One can call add_time/schedule_timer and del_timer from within
the callback.
diff --git a/configure.in b/configure.in
index 1c4070c..034972a 100644
--- a/configure.in
+++ b/configure.in
@@ -16,4 +16,10 @@
 
 dnl Checks for typedefs, structures and compiler characteristics
 
-AC_OUTPUT(include/openbsc/Makefile include/Makefile src/Makefile Makefile)
+AC_OUTPUT(
+    include/openbsc/Makefile
+    include/Makefile
+    src/Makefile
+    tests/Makefile
+    tests/timer/Makefile
+    Makefile)