fflush, fseeko: port to musl cross-compiles

fflush, fseeko: port to musl cross-compiles

* lib/fseeko.c (fseeko): Assume that fflushing stdin works if
on some implementation that (1) is not known to be buggy,
(2) claims conformance to POSIX.1-2008 or later, and (3) is being
cross-compiled to so we can't easily check for lack of
conformance.  This is for cross-compiling to musl.
Reported by Rich Felker in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-06/msg00043.html>.
* m4/fclose.m4 (gl_FUNC_FCLOSE):
* m4/fflush.m4 (gl_FUNC_FFLUSH):
* m4/fseeko.m4 (gl_FUNC_FSEEKO):
Adjust to above change.
* m4/fflush.m4 (gl_FUNC_FFLUSH_STDIN): Set gl_cv_func_fflush_stdin
to 'cross', not to 'no', when cross-compiling.  AC_DEFINE
FUNC_FFLUSH_STDIN to 1, 0, -1 if fflushing stdin is known to work,
known not to work, or unknown.