Only generate coverage targets if the COVERAGE option is enabled

[PJK]

Fixes #383

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cc954fd) to head (4077d50).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #384   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         1653      1653           
=========================================
  Hits          1653      1653           

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