WIP: Add small comments in Mac TPL build documentation

[chapman39]

I'm going through the Mac build documentation for my machine. So far I just added one comment.

I was getting thrown off by the documentation mentioning /opt/homebrew for the installation prefix, when it's /usr/local for Intel-based Macs. I'm new to using brew, so maybe I'm just a noob, but it did take me a bit to figure out.

[btalamini]

I think there is a command to query homebrew's prefix. Googling turns up brew --prefix. I don't have homebrew, so I can't check - can one of you brew users try? This could make the instructions more generic, as well as handle the case that a user installed homebrew in a non-default location

[chapman39]

@btalamini I added a comment about brew --prefix, thanks.

Rather referring to the assumed prefix /opt/homebrew, do we want something generic like ${BREW_PREFIX} in the documentation? I don't have a strong preference either way.

[chapman39]

I'm very confused why this is happening when I have +devtools. Has +devtools been tested on macos?

[exe: /Users/chapman39/dev/serac/serac_libs1/spack/bin/spack -D /Users/chapman39/dev/serac/serac_libs1/spack_env concretize --fresh ]
==> Error: concretization failed for the following reasons:

   1. Cannot select a single "version" for package "py-numpy"
[spack version: 0.22.0.dev0 (6d244b3f6776b3697861c6b5cebbee376c30bff3)
]

Only when I remove the python section from my spack.yaml, I'm able to concretize with +devtools.

[chapman39]

Looks like boundary condition boundary_cond test fails. I can look into this a bit more but wanted to report.

***********************************
[WARNING in line 67 of file /Users/chapman39/dev/serac/repo/src/serac/infrastructure/../../serac/physics/boundary_conditions/boundary_condition.hpp]
MESSAGE=Attempting to compare tags of two different enum types (always false)
***********************************

...

[  FAILED  ] 1 test, listed below:
[  FAILED  ] BoundaryCond.DirectTrueDofs

[btalamini]

@chapman39, since you asked, my preference would be to replace /opt/homebrew everywhere in this section with $(brew --prefix) since that will always do the right thing. But we can do that later. Let's just get this in.