This applies the same changes to the djgpp and mingw versions of
error_constants.h as r11-6137 did for the generic version.
All of these constants are defined as macros by <errno.h> on these
targets, so we can just test the macro directly instead of checking for
it at configure time.
libstdc++-v3/ChangeLog:
* config/os/djgpp/error_constants.h: Test POSIX errno macros
directly, instead of corresponding _GLIBCXX_HAVE_EXXX macros.
* config/os/mingw32-w64/error_constants.h: Likewise.
* config/os/mingw32/error_constants.h: Likewise.
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