Incorporate relevant env vars into build cachebusting, maybe

[evhan]

For example, if a user changes CSC_OPTIONS and runs chicken-install again, they presumably want things to be rebuilt. Same goes for CHICKEN_REPOSITORY_PATH, etc.

But then, there are a bunch of other variables that affect builds as well. Where does it end?

It's probably worth considering whether there is a way to consider some environment variables to make caching behaviour more correct, without going totally overboard. For example, we know the set of CHICKEN-defined environment variables that affect builds, that could be where we draw the line. But this needs more thought first.

[felix winkelmann]

See current-status in egg-compile.scm. Any changes to those variables will cause rebuild.