Autotools integration

[mikebentley15]

Feature Request

Describe the new feature:
Right now, FLiT only has GNU Make integration and experimental Ninja Build integration.

What about projects using Autotools? As it is now, we could have the flit-config.toml and custom.mk be input files that Autotools can populate (i.e., flit-config.toml.in and custom.mk.in). I think this is the only reasonable approach, but I'm not too familiar with how Autotools works.

Suggested change:
Somehow make it easy to generate flit-config.toml and custom.mk from an Autotools setup. Perhaps there's a way to define functions with Package Config. Perhaps just document how users can do it with their own *.in files. I'm not sure.