message: Use -O0 with NDEBUG for coverage builds.
  We don't want debug code in the coverage stats.
diff: @@ -6,21 +6,26 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
  endif()
  endif()
  +option(ENVOY_DEBUG "build debug binaries" ON)
  option(ENVOY_CODE_COVERAGE "build with code coverage intrumentation" OFF)
  +
  if (ENVOY_CODE_COVERAGE)
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --coverage")
  add_definitions(-DCOVERAGE)
  endif()
  -option(ENVOY_DEBUG "build debug binaries" ON)
  -if (ENVOY_DEBUG)
  - set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O0")
  +if (ENVOY_DEBUG AND NOT ENVOY_CODE_COVERAGE)
  add_definitions(-DDEBUG)
  else()
  - set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2")
  add_definitions(-DNDEBUG)
  endif()
  +if (ENVOY_DEBUG OR ENVOY_CODE_COVERAGE)
  + set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O0")
  +else()
  + set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2")
  +endif()
  +
  option(ENVOY_SANITIZE "build with address sanitizer" OFF)
  if (ENVOY_SANITIZE)
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address")
  