Fix --enable-linux-builtin

Adding VPATH support, commit 47a4a6f, required that a src
and obj line be added to the top of the Makefiles. They
must be removed from the Makefiles when builtin.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue zfsonlinux/spl#481
Issue zfsonlinux/spl#498