Avoid fiddling with autoconf internals and use AC_DEFINE_UNQUOTED to
define macros in the configuration headers rather than handcoding an
equivalent shell sequence with the use of the `as_echo' undocumented
variable.
Similarly use AC_MSG_ERROR for error handling rather than the internal
undocumented `as_fn_error' variable. Switch to using 1 as the exit code
as it makes no sense to refer $? in the contexts involved, it's not a
command failure handled there.
