std::make_unique is a very useful part of the new C++ smart pointers
ecosystem, as it allows one to dispense entirely with "new" and also
provides exception safety in some cases when creating temporary
variables.
It also allows more concise code by avoiding repetition of the type:
std::make_unique<TYPE>( args );
vs
std::unique_ptr<TYPE>( new TYPE( args ) );
This commit adds a "polyfill" to provide std::make_unique when C++11 is
enabled. The implementation is that submitted to the C++ committee, and
is essentially how it is done in GCC for C++14.
The intention is to allow KiCad to use this implementation when using
C++11 and when C++14 or greater is a requirement, to remove this code
and use the compiler implementation.