Make tup's build more configurable

[kalrish]

The goal of this pull request is to make it easier to change the flags with which tup itself is built. Previously, doing that required tweaking the Tupfiles themselves; with these commits, a level of configurability is supported via tup.config, which the user can provide. This way is better not only because it separates the what to build from the how to do it, but also because Kconfig syntax poses a lower entry level for those wanting to build tup but unfamiliar with it (i.e. packagers).

Being able to change build flags is desirable for several reasons. It's also required when targetting various platforms, and it frees us from the burden of providing the right flags for every situation.

In case no configuration is specified, the previous flags are used. I have tested building with no configuration on Windows, and it works.

To help users and contributors, a couple config templates are supplied.