Figure out reasonable PREFIX settings for distributions

[acfoltzer]

#127 gave us a more flexible way to configure paths, we just need to make effective use of it in distributions.

There are a few scenarios:

1. a developer wants to build a distribution and run it in place
2. we want to distribute a tarball that can be extracted to any location on a filesystem
3. we want to distribute a package manager-specific artifact like a .rpm or Homebrew formula

Right now the Makefile optimizes for case 3 by baking in /usr/local/ or C:\Program Files\... into the make dist target. We need to make it possible to support cases 1 and 2 as well.

1 and 2 are likely to end up looking very similar (i.e., a distribution directory with /bin/, /share/cryptol/, etc). However we may want to add some sort of conditional compilation step so that those paths are not baked into the module search path; instead we should just rely on our heuristics for determining paths relative from the executable to avoid accidentally bringing in system-wide stuff that doesn't belong to us.

[acfoltzer]

This is in thanks to the relocatable cabal flag and the associated Makefile machinery to drive it.