Make spl directory setable when building rpms and add --buildroot

This adds ability to set the location of spl via defines when
building from the spec files. This is useful for build systems
that build spl and zfs together without installing the actual rpms.

Signed-off-by: Nathaniel Clark <Nathaniel.Clark@misrule.us>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1486