Make copyObject work in C++

Calling copyObject fails in C++ with an error like in most setups:

error: use of undeclared identifier 'typeof'; did you mean 'typeid'

This is due to the C compiler supporting used to compile postgres
supporting typeof, but that function actually not being present in the
C++ compiler. This fixes that by using decltype instead of typeof when
including the header in C++.

Realized because of Thomas' not about how much of our headers should
work in C++, and remembering I hit this specific problem myself.

Another approach would be to force the value of HAVE_TYPEOF to 0 if __cplusplus.