build: switch to C++14

Replace our homegrown implementation of optional (based
on Boost.Optional) with optional-lite by Martin Moene.

Change-Id: I4632cbc7c705a94af0b85a335515e3430bf3cb1f
Refs: #3076
diff --git a/src/mgmt/status-dataset-context.cpp b/src/mgmt/status-dataset-context.cpp
index 2abbaa7..c8671cd 100644
--- a/src/mgmt/status-dataset-context.cpp
+++ b/src/mgmt/status-dataset-context.cpp
@@ -86,7 +86,7 @@
                    makeBinaryBlock(tlv::Content, m_buffer->buf(), m_buffer->size()),
                    m_expiry, false);
 
-      m_buffer = std::make_shared<EncodingBuffer>();
+      m_buffer = make_shared<EncodingBuffer>();
     }
   }
 }