message: adding 2 more warnings for GCC 7
diff: @@ -122,15 +122,17 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
  add_compiler_flags(-Wvector-operation-performance)
  endif()
  - if(CMAKE_CXX_COMPILER_VERSION VERSION_EQUAL 6.0 OR CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 6.0)
  + if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 6.0)
  add_compiler_flags(-Wshift-overflow=2)
  add_compiler_flags(-Wnull-dereference)
  add_compiler_flags(-Wduplicated-cond)
  endif()
  - if(CMAKE_CXX_COMPILER_VERSION VERSION_EQUAL 7.0 OR CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 7.0)
  + if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 7.0)
  add_compiler_flags(-Walloc-zero)
  add_compiler_flags(-Walloca)
  + add_compiler_flags(-Wduplicated-branches)
  + add_compiler_flags(-Wrestrict)
  endif()
  endif()
  