Build configuration cache mode

[griwes]

Create an (optional) mode that'll cache all configuration options in an output directory for further use, CMake-style.

This is an useful feature if you're passing a lot of options to the configuration and want to save them, but I do not believe this is essential enough to be the default (vs how convenient it is to just change $CXXFLAGS for a moment to test something) or even a part of MVP, but I think it's important enough to be present.