Do not add the content of a file if it's a symlink.

This wasn't necessary for TAR-like (TGZ, TBZ2, etc...) archive
because for those the size was 0. Either there is an error in
upstream libarchive concerning the size or we should not rely
on size of the entry for adding content.

Source/cmArchiveWrite.cxx

@@ -247,10 +247,14 @@ bool cmArchiveWrite::AddFile(const char* file,
     return false;
     }
 
-  // Content.
-  if(size_t size = static_cast<size_t>(archive_entry_size(e)))
+  // do not copy content of symlink
+  if (!archive_entry_symlink(e))
     {
-    return this->AddData(file, size);
+    // Content.
+    if(size_t size = static_cast<size_t>(archive_entry_size(e)))
+      {
+      return this->AddData(file, size);
+      }
     }
   return true;
 }