Overridden package variables not available to descendants

[tskisner]

Overridden package variables are specified in the per-system make file snippets. These are used to build module files for the overridden packages which prepend to CPATH, LIBRARY_PATH, etc and propagate any variables in the system make file. The internal hpcports build system does not use these modules, instead relying on the per-package shell snippets which are sourced. No such shell snippets exist yet for overridden packages.