Properly ignore bdi_setup_and_register return value in autotools check

This broke compilation against Linux 3.13 and GCC 4.7.3. zfsonlinux/spl#312 is required to test this, but the reverse is not necessarily true. This issue is more likely attributable to GCC 4.7.3 than Linux 3.13, although I did not check that possibility.

Signed-off-by: Richard Yao <ryao@gentoo.org>