message: cmake: Allow unknown attributes in Clang
diff: @@ -51,6 +51,7 @@ cable_add_cxx_compiler_flag_if_supported(-Wunreachable-code-break)
  cable_add_cxx_compiler_flag_if_supported(-Wduplicated-cond)
  cable_add_cxx_compiler_flag_if_supported(-Wduplicate-enum)
  cable_add_cxx_compiler_flag_if_supported(-Wlogical-op)
  +cable_add_cxx_compiler_flag_if_supported(-Wno-unknown-attributes)
  if(CMAKE_CXX_COMPILER_ID MATCHES Clang)
  option(WEVERYTHING "Enable almost all compiler warnings" OFF)
  