Kernel header installation should respect --prefix

This enables preliminary support for building Gentoo's ZFS packaging on
other Linux systems via Gentoo Prefix.

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>
Search /usr/local/src for SPL Object Directory

Since we changed the default location for the kernel headers to respect
--prefix in the SPL, we must search that location to prevent user builds
from breaking.

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>