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>

src/backoff_entry.h

diff --git a/src/backoff_entry.h b/src/backoff_entry.h
index 2df0d8a..002fb8d 100644
--- a/src/backoff_entry.h
+++ b/src/backoff_entry.h
@@ -57,7 +57,7 @@
   // Lifetime of policy must enclose lifetime of BackoffEntry. The
   // pointer must be valid but is not dereferenced during construction.
   explicit BackoffEntry(const Policy* const policy);
-  virtual ~BackoffEntry() = default;
+  virtual ~BackoffEntry() {}
 
   // Inform this item that a request for the network resource it is
   // tracking was made, and whether it failed or succeeded.