Coverage should be calculated without optimizations

[neusdan]

otherwise the report is unprecise.

Also described here:
https://gcc.gnu.org/onlinedocs/gcc/Gcov-and-Optimization.html

...if you want to prove that every single line in your program was executed, you should not compile with optimization at the same time.

[codecov-io]

Current coverage is 82.04%

Merging #536 into master will decrease coverage by 0.03%

@@             master       #536   diff @@
==========================================
  Files           307        307           
  Lines         23537      27173   +3636   
  Methods           0          0           
  Messages          0          0           
  Branches         51         51           
==========================================
+ Hits          19321      22295   +2974   
- Misses         4191       4853    +662   
  Partials         25         25           

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