message: core: fix fallback CV_CONSTEXPR definition
  keep it empty to avoid failures of these declarations:
  static CV_CONSTEXPR const int var = ...;
diff: @@ -691,7 +691,7 @@ __CV_ENUM_FLAGS_BITWISE_XOR_EQ (EnumType, EnumType)
  # endif
  #endif
  #ifndef CV_CONSTEXPR
  -# define CV_CONSTEXPR const
  +# define CV_CONSTEXPR
  #endif
  // Integer types portatibility
  