waf: fix tri-state of --with-sendfile-support being auto the default

waf: fix tri-state of --with-sendfile-support being auto the default

--with-sendfile-support should be used automatically if it's found.

Configure now also fails reliably for all platforms when sendfile was
explicitly requested but failed to be detected.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=9707

Signed-off-by: Bjoern Jacke <bjacke@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Jan  8 07:03:49 CET 2019 on sn-devel-144