Add support for build flavours

[snowleopard]

The make-based build system supports a number of build flavours, collections of settings tailored for a particular use-case, e.g. quick, validate, etc.

Relevant issues: #132, #186, #187.

[snowleopard]

I put build flavours in separate files (ghc/ghc@2c24fd7), so I could more easily diff them, and spot inconsistencies. YMMV

@thomie Let me respond to your comment here.

Yes, for now I am also putting different flavours in separate files. Let's see how small/big these files will eventually become. We probably wouldn't want to have multiple files each containing a single useful line and a dozen of boilerplate imports etc., but maybe this won't happen.

[thomie]

As discussed in #243, it would be nice to be able to select a BuildFlavour from within User.hs.

Other setting in User.hs should then override settings predefined by that BuildFlavour. And command-line flags should yet again override all the previous settings.

[snowleopard]

The relevant bits are here:

data Flavour      = Default | Quick deriving (Eq, Show)

-- | Default values for 'CmdLineFlag.Untracked'.
defaultUntracked :: Untracked
defaultUntracked = Untracked  { flavour = Default, ... }

And here:

flavourArgs :: Args
flavourArgs = mconcat
    [ cmdFlavour == Quick ? quickFlavourArgs ]

getArgs :: Expr [String]
getArgs = fromDiffExpr $ mconcat [ defaultBuilderArgs
                                 , defaultPackageArgs
                                 , flavourArgs
                                 , userArgs ]

The trick is to implement the overriding of defaults in Settings/User.hs without introducing import cycles... Not sure how to do that yet.

[snowleopard]

I implemented build flavours while working on #268, so I'm closing this issue.