Default PETSc BLAS/LAPACK to vecLib on macOS

[boyceg]

Add an option to set the PETSc BLAS/LAPACK backend to veclib on macOS, and make this the default if --lapack-library is not explicitly set.

[boyceg]

@drwells I think we want to default to using https://developer.apple.com/documentation/accelerate/veclib on MacOS.

[boyceg]

@drwells: see if this addresses your comments.

AFAIK, the official way to link to vecLib is to use -framework Accelerate, but I am not sure if we can use strings with spaces here.

[drwells]

It looks like PETSc can detect veclib in its configuration scripts, but it does basically the same thing as this so if this works I wouldn't worry about it.

Is veclib always installed on macs? We should add some documentation to the readme explaining that it is a dependency on macs (and users can get around it by specifying openblas instead).

[boyceg]

AFAIK, Accelerate and vecLib have been around forever and are always installed. I am not sure why we haven't been using them already.

[drwells]

I'd expect that if we do nothing then PETSc will detect them on macs, so we may already be using them.

[boyceg]

I'd expect that if we do nothing then PETSc will detect them on macs, so we may already be using them.

Unclear. I just tried recompiling the current PETSc release on my Mac, and I see:

BlasLapack:
  Libraries:  -llapack -lblas
  Unknown if this uses OpenMP (try export OMP_NUM_THREADS=<1-4> yourprogram -log_view)
  uses 4 byte integers

From what I've seen, I don't think that is how you link to vecLib.