Ignore exit status from diff.
* configure.in (enable_check_abi): Document possible value "warn".
Change default to no for now.
* configure: Regenerated.
* sysdeps/unix/Makefile ($(objpfx)stub-syscalls.c): Emit stub_warning
macro calls and a #include <stub-tag.h> at the end.
* Makerules ($(objpfx)stubs): Tweak sed commands.
* sysdeps/unix/sysv/linux/syscalls.list: Use - rather than EXTRA in
caller column for *xattr syscalls, since they are in sysdeps/generic.