Move spl.release generation to configure step

Previously, the spl.release file was created at 'make install' time.
This is slightly problematic when the file is needed without running
'make install'. Because of this, the step creating the file was removed
from 'make install' and replaced with a more appropriate spl.release.in
file.

As a result, the spl.release file will now be created earlier as part
of the 'configure' step as opposed to the 'make install' step.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #135