ENH: Error if GENERIC_IO_INCLUDES isn't defined

Modified Makefile s.t. an error is thrown in the terminal if
GENERIC_IO_INCLUDES does not exist in the environment. Previously,
the build would fail, but it was not obvious what the real problem
was.
ENH: Error if GENERIC_IO_INCLUDES isn't defined

Modified Makefile s.t. an error is thrown in the terminal if
GENERIC_IO_INCLUDES does not exist in the environment. Previously,
the build would fail, but it was not obvious what the real problem
was.

git-svn-id: https://svn.alcf.anl.gov/repos/DarkUniverse/hacc/trunk/cosmotools@1437 952bdfbe-fa2e-45c0-a9ae-ac90d4a62daf
Merge branch 'svn/ensure-genericio-is-specified'