Various mac issues on Sequoia 15.4 on an M4

[teuben]

I'm using brew to get some 3rd party things going: wget, gfortran, libxext, the special pgplot (see #175 )

gcc is now clang v17, and the compiler is giving more warnings which no doubt will eventually cause code rot.

All of this spurred by the arrival of a new macbook air M4 13 inch (spring 2025 version)

When this is all working, a new NEMO version should be released.