Use a specific commit of Heap-Layers

[ryandesign]

Looks like Hoard's Makefile and GNUmakefile clone master of Heap-Layers. Unless you never change Heap-Layers, it seems likely that at some point you will introduce a change to Heap-Layers that will require a corresponding change to Hoard. When that happens, old versions of Hoard (which might be present in package management systems) will no longer be able to build. This problem might not always be noticed right away, leaving your software in broken state in that package management system for an unbounded period of time.

For this reason, you should download a specific version of Heap-Layers that is known to work with this version of Hoard.

I think git already behaved that way before when you were using submodules, so you could get this behavior back by reverting 5275686 and also making the same change to GNUmakefile.

[emeryberger]

Submodules have been a problem; people repeatedly fail to recursively clone and then wonder why Hoard won't build. It's a headache.

I could, however, checkout a specific version of Heap-Layers. Will that address this concern?