[Bug libstdc++/93151] system_error header fails to compile with -D_XOPEN_SOURCE=600

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93151

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
POSIX doesn't allow that. If such systems exist, they should provide their own
config/os/*/error_constants.h header with the correct set of constants.

The generic/error_constants.h file should assume POSIX.