macOS: Conditionally add -Wl,-ld_classic for Xcode CLT >= 15

[DilumAluthge]

Replaces #935 (closes #935)
Reference #738

---

This is based on #935. All credit goes to @PhilReinhold for providing the fix and @jayscook for opening the original PR (#935).

This PR differs slightly from #935. #935 used a try-catch. This PR uses ignorestatus() to ignore non-zero exit codes from running the pkgutil ... command, and uses isnothing() to handle the case where the regex does not match. Also, this PR factors out the "detect CLT major version" functionality out into a separate helper function.

Also, this PR now addresses #935 (comment)

We do not add these flags for CLT versions <= 14.

[DilumAluthge]

This PR doesn't yet address #935 (comment), but I'll look into it further.

Okay, #935 (comment) is now addressed by the combination of c9a4b52 and adf5737.

[codecov]

Codecov Report

Attention: Patch coverage is 48.64865% with 19 lines in your changes missing coverage. Please review.

Project coverage is 83.00%. Comparing base (85cdb03) to head (90931fe).

Files with missing lines	Patch %	Lines
src/xcode.jl	43.75%	18 Missing ⚠️
src/PackageCompiler.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #972      +/-   ##
==========================================
- Coverage   84.56%   83.00%   -1.57%     
==========================================
  Files           3        4       +1     
  Lines         823      859      +36     
==========================================
+ Hits          696      713      +17     
- Misses        127      146      +19     

☔ View full report in Codecov by Sentry.
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[DilumAluthge]

Okay, #935 (comment) is now addressed by the combination of c9a4b52 and adf5737.

[DilumAluthge]

Specifically, we now run $cc --version, and we use that output to determine whether or not the compiler is the Clang from Xcode.

[DilumAluthge]

It's not entirely clear to me if we still need this PR, because #930 seems to have fixed #738, at least in the macOS CI on this repo.

[jayscook]

Thanks for putting this together @DilumAluthge!

[DilumAluthge]

@jayscook Thank you for putting the original PR together (and thank you to @PhilReinhold for writing the original code).

[DilumAluthge]

@topolarity Could you take a look at this PR, and let me know if it addresses your concerns in #935 (comment) and #935 (comment)?

[topolarity]

Sure thing! Looks like the right direction, but I won't be able to take a close look until Monday

[DilumAluthge]

Sounds good, thanks man!

[DilumAluthge]

@sjkelly I see you also reviewed the original PR (#935), so I've requested your review on this one.