chosing a C++ version/compiler (was: icu wants c++11)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> I think we need to move on first-class support for c++ flavors.
Yes.

> I think we are going to have to choose a compiler version per OS/branch.
pkgsrc should be chosing a default compiler version per OS/branch just like 
it choses a default Python or PHP or whatnot version.
The default should be overrideble in mk.conf and the infrastructure should 
error out if a package requires an incompatible compiler version (e.g., a 
language level not supported by the chosen compiler). Just like with PHP, 
Python or whatnot versions.

Of course pkgsrc needs a sensible replacement for ``I need GCC 8.15'' or 
``I need c++14/18'' first. I don't know enough about C++, but maybe something 
like ``I need at least C++ level 11'' and ``I'm incompatible with GCC 5.3'' 
will do.

Is C++n a superset of C++m for n>m?