when using =std=C++98 to complie, #include

when I use -std=c++98 to compile, the error occur

In file included from c:\mingw\lib\gcc\mingw32\5.3.0\include\c++\bits\locale_facets.h:39:0,
                 from c:\mingw\lib\gcc\mingw32\5.3.0\include\c++\bits\basic_ios.h:37,
                 from c:\mingw\lib\gcc\mingw32\5.3.0\include\c++\ios:44,
                 from c:\mingw\lib\gcc\mingw32\5.3.0\include\c++\ostream:38,
                 from c:\mingw\lib\gcc\mingw32\5.3.0\include\c++\iostream:39,
                 from 2299.cpp:4:
c:\mingw\lib\gcc\mingw32\5.3.0\include\c++\cwctype:89:11: error: '::iswblank' has not been declared
   using ::iswblank;

my g++ version is g++(GCC) 5.3.0. I want to know why......