[CMake] Don't confuse users by announcing that C++11 OPTIONAL features have not been found

The CMake configuration process always reported the C++11 features
tested for by dune-typetree as not found, even if they were. Fix the
problem by calling the appropriate CMake macro.
[PowerNode] Add constructor that takes a variadic list of shared_ptrs

When constructing highly nested trees, it is often convenient to
directly allocate and assign child nodes without creating intermediate
variables. Unfortunately, the PowerNode is missing a constructor that
allows you to do that.

Fixed by replicating the infrastructure for variadic lists of references.
[pkg-config] Add library to pkg-config file.

dune-typetree ships a library, so add the appropriate linker flags to
the pkg-config file.

Thanks to Markus Blatt, who fixed the same problem in dune-pdelab!
[Release] Update version information for 2.3.0-rc1
Merge branch 'releases/2.3'

Pull in fixes from release branch, most importantly support for passing
a variadic list of shared_ptrs to the PowerNode constructor.

* releases/2.3:
  [Release] Update version information for 2.3.0-rc1
  [pkg-config] Add library to pkg-config file.
  [PowerNode] Add constructor that takes a variadic list of shared_ptrs
  [CMake] Don't confuse users by announcing that C++11 OPTIONAL features have not been found

Conflicts:
	dune.module
	dune/typetree/powernode.hh