fwrite: silence __wur without using inline

fwrite: silence __wur without using inline

The libvirt folks reported[1] a link error of multiple rpl_fwrite
definitions that hits only when optimization and FORTIFY_SOURCE
are both enabled, due to improper use of inline.  But since that
particular use of rpl_fwrite exists only to work around a spurious
gcc warning on some versions of glibc, we can use gcc extensions
to acheive the same effect without using inline.  This approach
copies from ignore-value.h.

[1] https://lists.gnu.org/archive/html/bug-gnulib/2013-01/msg00014.html

* lib/stdio.in.h (fwrite): Limit warn_unused_result workaround to
just gcc, and in a way that avoids inline issues.
* modules/stdio (Depends-on): Drop extern-inline.

Signed-off-by: Eric Blake <eblake@redhat.com>