COVERAGE_CXX_COMPILER_FLAGS usage

HI, 
I don't get how the `COVERAGE_CXX_COMPILER_FLAGS` is used. It seems it's only set, but not used afterwards.

https://github.com/bilke/cmake-modules/blob/1fcf7f4a2179a7b649be15473906882871ef5f0b/CodeCoverage.cmake#L174C7-L174C89

Any explaination welcome.