unix: don't explicitly instantiate v8::Persistent<x> templates

These explicit instantiations were added to make MSVC happy. It turns
out that some older versions of gcc and llvm now complain about duplicate
symbols, so we instantiate these templates only when MSVC is used.

src/node_object_wrap.h

@@ -29,8 +29,11 @@
 // Explicitly instantiate some template classes, so we're sure they will be
 // present in the binary / shared object. There isn't much doubt that they will
 // be, but MSVC tends to complain about these things.
+#ifdef _MSC_VER
   template class NODE_EXTERN v8::Persistent<v8::Object>;
   template class NODE_EXTERN v8::Persistent<v8::FunctionTemplate>;
+#endif
+
 
 namespace node {