compute_plan now explicitly returns an IV.

This makes sure the pointer is correctly encoded
Makefile.PL now uses 'INC' for all its C flags instead of 'CCFLAGS'

It turns out that ExtUtils::MakeMaker forgets all of its default C flags if you
touch the CCFLAGS key, even if you only append to it. These missing flags are
sometimes important, causing odd errors. I now add all my options to the 'INC'
key, which makes things work