libhookgetenv: Trivial rephrase of comment.
Add some new functionality to depset and dict.

In preparation for noting `stat` as a pseudo-input. See issue #4.
Rephrase set iteration as a foreach with a callback.

Given support for nested functions, we can do this much more elegantly.
Rephrase dictionary operations the same way as set.
Convert build system to GNU Make.

CMake was gaining us very little when this tool is not cross-platform anyway.
In reflection, CMake's syntax is not appreciably more robust than GNU Make's
and, if anything, it is more difficult to latch on to libraries because of
CMake's comparative immaturity in this arena.
Squash some warnings.

Interestingly this points to the fact that CMake was not using the options I
thought it was.