Hadrian: introduce userDefaultFlavour, making default flavour overridable
This patch introduces the userDefaultFlavour user setting. It should
be the name of the default flavour to use when no --flavour argument is
passed. Before this patch, we would just always default to... the
default flavour. With this patch, we default to whatever Flavour whose
name is userDefaultFlavour, therefore providing a way for users to
"persist" their choice of flavour, not having to repeat --flavour=[...]
in every hadrian command.
Set userDefaultFlavour = "quickest", run hadrian/build.sh, check
that the quickest flavour is indeed picked.
Reviewers: snowleopard, bgamari
Reviewed By: snowleopard
Subscribers: mpickering, rwbarton, carter
GHC Trac Issues: Trac #15890
Differential Revision: https://phabricator.haskell.org/D5454