Update method of detecting and linking quadmath

[mborland]

Fixes: #191

The next step in the never ending battle to get this correct. @pdimov Do you see any potential issues with this? I replaced check_cxx_source_compiles with check_cxx_source_runs due to Reuben's previous issue (#176) where the header existed, but the compiled library did not.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.01%. Comparing base (007ac06) to head (f2d2aca).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #193   +/-   ##
========================================
  Coverage    94.01%   94.01%           
========================================
  Files           66       66           
  Lines         8439     8439           
========================================
  Hits          7934     7934           
  Misses         505      505           

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[pdimov]

This looks OK to me, by visual inspection alone.

[mborland]

Looking at the old CI runs the posix-cmake-test suite all yielded -- Boost.Charconv: quadmath support OFF. Now the macOS runs show -- Boost.Charconv: quadmath support OFF as they should, and Ubuntu runs show -- Boost.Charconv: quadmath support ON.