* lib/autoconf/functions.m4 (AC_FUNC_FSEEKO): Don't compile the

* lib/autoconf/functions.m4 (AC_FUNC_FSEEKO): Don't compile the
fseeko testing program twice; just use the earlier result.
* lib/autoconf/specific.m4 (_AC_SYS_LARGEFILE_MACRO_VALUE):
Set cache var to 'unknown' (not 'no') if leaving the macro unset
still doesn't let the program compile.
(AC_SYS_LARGEFILE): Test for _LARGE_FILES only if earlier tests
failed.

* lib/autoconf/functions.m4: Fix problems reported by Ralf Wildenhues.
(AC_FUNC_ERROR_AT_LINE): Don't bother to check for error.h.  Just
include it, without including anything else.
(AC_FUNC_FSEEKO): Avoid gcc -Wall warnings about constant
expressions.
(AC_FUNC_STRNLEN): Require AC_USE_SYSTEM_EXTENSIONS.

* lib/autoconf/functions.m4 (AC_FUNC_ERROR_AT_LINE): Check for
`error.h', and include it, for a `error_at_line' prototype.
Use a nonempty format string in the link test.
* lib/autoconf/functions.m4 (AC_FUNC_WAIT3): Include <sys/wait.h>,
for a declaration of wait3.