use 'port select gcc' for OSX macports settings

[Roland Haas]

@Barry Wardell suggested to use port select gcc similar to the already used port select mpi to select a default gcc executables. This means that the compiler can be called gcc rather than gcc-8 and one is closer to be able to use the generic.* files on OSX + MacPorts as well.

[Steven R. Brandt]

DId this fix work for our student? Is it a proposed change for the Tutorial?

[Roland Haas]

Robyn needed an darwin18.6.0 architecture file, and update package names in port install and port select, as well as updating osx-macports.cfg to account for the new package names.
This change alone would not have helped (the architecture file is still needed). This is a proposed change for the tutorial which I am currently test running on a Mac VM.

[Roland Haas]

Applied as git hash aad9bd2 "CactusTutorial: use gcc select to set default gcc compiler" of jupyter-et