Test for errno distinctness dynamically rather than statically,
since the static test blows up the compiler on Hurd.
Problem reported by Bruno Haible in:
https://lists.gnu.org/r/bug-gnulib/2024-08/msg00039.html
Also, test that errno values can all be used in #if,
and improve diagnostics.
* tests/test-errno.c: Include stdio.h, stdlib.h, string.h.
(USABLE_IN_IF): New macro. Use it to check errno values in #if.
(ERRTAB): New macro.
(struct nameval): New type.
(errtab): New global variable.
(errtab_cmp): New function.
(main): Test for errno distinctness dynamically not statically.
Diagnose lack of distinctness better.