Accept user specified spl and kernel source dir.

I've issued this patch several times, but this time I'd make it a simple one, not a combination of many fixes.


Having '/usr/src/spl-VERSION' hardcoded (and almost the only choice) is a BAD THING (tm). This patch will accept the values issues with a previous './configure --with-spl=... --with-spl-obj=...' and generate propper zfs-kmod.spec file that is usable without any softlinks or installs of spl before hand.