Add the possibility to use shared trivial types as deleters for objects managed
by std::unique_ptr / UniquePtr.

GCC does not accept an extern "C" storage class specifier along with
definition of the global variable. Split declaration and definition to
overcome this.

Apparently MacOS's unique_ptr implementation puts the
deleter as the 2nd member. Reflect this in the unique_ptr definition.

* Add unnamed lifetimes to drop implementation for UniquePtrTarget
* Update expected stderr to include the deleter type parameter.