Add -Wextra-semi -Wundefined-func-template to the default CXXFLAGS

And fix the resulting compilation errors.

Change-Id: I16f679836a0cf2a3ff5dfcf6f6b67bfbfe4cdbd7
Refs: #4248
diff --git a/src/selectors.cpp b/src/selectors.cpp
index a207775..80c891f 100644
--- a/src/selectors.cpp
+++ b/src/selectors.cpp
@@ -109,11 +109,7 @@
   return totalLength;
 }
 
-template size_t
-Selectors::wireEncode<encoding::EncoderTag>(EncodingImpl<encoding::EncoderTag>& encoder) const;
-
-template size_t
-Selectors::wireEncode<encoding::EstimatorTag>(EncodingImpl<encoding::EstimatorTag>& encoder) const;
+NDN_CXX_DEFINE_WIRE_ENCODE_INSTANTIATIONS(Selectors);
 
 const Block&
 Selectors::wireEncode() const