(strerror_r): Do not declare unless !_LIBC.

(strerror_r): Do not declare unless !_LIBC.
Do not check for HAVE_DECL_STRERROR_R missing unless STRERROR_R_CHAR_P.
Use strerror_r that is only a macro, even if it is not a function.
(strerror): Check for HAVE_DECL_STRERROR before declaring.
(private_strerror): Use prototypes, not old-style function definition.
(print_errno_message): New function.
Support the POSIX 'int'-flavored strerror_r, as well as the traditional
char*-flavored one.
(error_tail, error, error_at_line): Use it.