Let the underlying functions call ‘exit’, instead of having the
Gnulib replacement macros do it. Use ‘unreachable’ to tell the
compiler that those functions exit when the status is nonzero.
This saves a function call.
* lib/error.in.h: Include stddef.h, not stdlib.h.
(__gl_error_call): Rely on the function to exit, using
‘unreachable’ to tell the compiler that the function does not return.
* modules/error (Depends-on): Add stddef.