Fix GCC 4.7 errors

GCC 4.7.2 fail overriding constructors defined with virtual and default.
Debian build also fails linking  without -lrt.
INT64_C is not defined on Debian with 4.7.2. "ll" should be enough.

Change-Id: Ia779f094ed64fc3b1941429485e81520d017c2f5
Reviewed-on: https://weave-review.googlesource.com/1671
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
diff --git a/src/commands/cloud_command_update_interface.h b/src/commands/cloud_command_update_interface.h
index 9538960..ed3aa7a 100644
--- a/src/commands/cloud_command_update_interface.h
+++ b/src/commands/cloud_command_update_interface.h
@@ -21,7 +21,7 @@
                              const DoneCallback& callback) = 0;
 
  protected:
-  virtual ~CloudCommandUpdateInterface() = default;
+  virtual ~CloudCommandUpdateInterface() {}
 };
 
 }  // namespace weave
diff --git a/src/commands/command_instance.h b/src/commands/command_instance.h
index 60620a1..32a93a9 100644
--- a/src/commands/command_instance.h
+++ b/src/commands/command_instance.h
@@ -37,7 +37,7 @@
     virtual void OnStateChanged() = 0;
 
    protected:
-    virtual ~Observer() = default;
+    virtual ~Observer() {}
   };
 
   // Construct a command instance given the full command |name| which must