Fix cmake -DEVENT__COVERAGE=ON

Fix cmake -DEVENT__COVERAGE=ON

- do not use compiler check from the root cmake rules with syntax error,
  let CodeCoverage check it
- fix CodeCoverage to check C compiler not CXX
- case insensitive checking of CMAKE_BUILD_TYPE
- replace flags with --coverage, and fix flags with linking with
  --coverate, otherwise it will not compile during checking flags and
  fails.