Add INCLUDE_UNIQUE_PTR and use it (PR bootstrap/82610)

Add INCLUDE_UNIQUE_PTR and use it (PR bootstrap/82610)

gcc/ChangeLog:
	PR bootstrap/82610
	* system.h: Conditionally include "unique-ptr.h" if
	INCLUDE_UNIQUE_PTR is defined.
	* unique-ptr-tests.cc: Remove include of "unique-ptr.h" in favor
	of defining INCLUDE_UNIQUE_PTR before including "system.h".

include/ChangeLog:
	* unique-ptr.h: Make include of <memory> conditional on C++11 or
	later.

From-SVN: r254024