Move Xcode CRuby settings into xcconfig

[johnfairh]

Time to revamp the Ruby settings and glue.

Update CRuby to hold an xccconfig to have all the custom settings - I suppose I didn't know what xcconfig was back in the day.

Add xcodebuild tests to CI.

Remove refs to -fdeclspec now no longer needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.08%. Comparing base (a20885b) to head (18bb2ec).
Report is 5 commits behind head on main.

❗ Current head 18bb2ec differs from pull request most recent head 9c380d4. Consider uploading reports for the commit 9c380d4 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   98.06%   98.08%   +0.01%     
==========================================
  Files          22       22              
  Lines        2068     2088      +20     
==========================================
+ Hits         2028     2048      +20     
  Misses         40       40              

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