Artem Pyanykh authored Sep 11, 2019 and Marge Bot committed Sep 23, 2019

Currently, if you change these ^ flavour parameters, rebuilding is not
triggered, since `programContext` doesn't set up a dependency on
those values.

Exposing these values via an oracle does set the dependency and
properly triggers a rebuild of binaries.

Several attempts to factor out these actions ended up in cyclic
dependency here or there. I'm not absolutely happy with this variant
either, but at least it works.

====

Issue repro:

In UserSettings.hs:
```
dbgDynamic = defaultFlavour { name = "dbg-dynamic"
                            , dynamicGhcPrograms = pure True,
                            ... }

dbgStatic = defaultFlavour { name = "dbg-static"
                           , dynamicGhcPrograms = pure False
                           ... }
```

Then in console:

```
$ hadrian/build.sh -j --flavour=dbg-dynamic
... does the build
$ hadrian/build.sh -j --flavour=dbg-static
... does nothing, considers binaries up to date
```