COMP: Further changes to the use of std::copy_n

1. Add preprocessor test for condition (detected by David
Doria) where ITK was configured & built with a compiler that
supports C++11 std::copy_n, but a user program was not.

2. Small changes suggested by Bradley Lowencamp that didn't
make it into the last patch.

Change-Id: I1635cccc52b2dbf3621b896db9a139f33cda4fe2
Merge topic 'RefineStdCopyNUsage'

137b5c3 COMP: Further changes to the use of std::copy_n